diff --git a/lib/http_tools.sh b/lib/http_tools.sh
index 813071c..1b587c4 100755
--- a/lib/http_tools.sh
+++ b/lib/http_tools.sh
@@ -23,11 +23,12 @@ fi
 
 #######################################################################
 #
-# Given a web resource and a cache, if the resource is cached, request 
-# the resource using HTTP Conditional GET [RFC 7232], otherwise issue
-# an ordinary GET request. In either case, if the server responds with 
-# 200, cache the resource and return the response body. If the server 
-# responds with 304, return the cached resource instead.
+# Given a web resource and a local cache, if the resource is cached, 
+# request the resource using HTTP Conditional GET [RFC 7232], otherwise 
+# issue an ordinary GET request for the resource. In either case, if 
+# the server responds with 200, cache the resource and return the 
+# response body. If the server responds with 304, return the cached 
+# resource instead.
 #
 # Usage: conditional_get [-v] [-F | -C] -d CACHE_DIR -t TMP_DIR HTTP_LOCATION
 #
@@ -45,11 +46,12 @@ fi
 # Use option -F or -C to alter the default behavior of the function.
 #
 # Option -F forces the return of a fresh resource, that is, if the
-# server responds with 304, an error occurs and the function returns
-# with a nonzero return code.
+# server responds with 304, the function returns with a nonzero return 
+# code.
 #
 # Option -C causes the function to go directly to cache. No GET request
-# is issued. This option is useful in offline mode.
+# is issued. (This option is useful in offline mode.) If the resource
+# is not cached, the function returns with a nonzero return code.
 #
 # The output of the curl command-line tool is stored in the following 
 # temporary files:
@@ -62,7 +64,22 @@ fi
 #
 # command_paths.sh
 #
-# This library file must be sourced BEFORE calling this function.
+# The library file must be sourced BEFORE calling this function.
+#
+# RETURN CODES
+#
+#    0: success
+#    1: quiet failure mode:
+#       option -F but no fresh resource available OR
+#       option -C but no cached resource available
+#    2: initialization failure
+#    3: unspecified failure
+#    4: hash operation failed
+#    5: curl failed
+#    6: call to get_header_value failed
+#    7: call to get_response_code failed
+#    8: copy to cache failed
+#    9: unexpected HTTP response
 #
 # TODO:
 #   - follow redirects?
@@ -72,15 +89,16 @@ fi
 conditional_get () {
 
 	if ! $COMMAND_PATHS; then
-		echo "ERROR: global command paths not found" >&2
+		echo "ERROR: $FUNCNAME: global command paths not found" >&2
 		return 2
 	fi
 	
-	local script_version="0.6"
+	local script_version="0.7"
 	local user_agent_string="HTTP Conditional GET client $script_version"
 	
 	local hash
 	local exit_code
+	local return_code
 	local cached_header_file
 	local cached_content_file
 	local conditional_get_mode
@@ -188,8 +206,8 @@ conditional_get () {
 	)
 	exit_code=$?
 	if [ $exit_code -ne 0 ]; then
-		echo "ERROR: $FUNCNAME failed to hash the location URL" >&2
-		return $exit_code
+		echo "ERROR: $FUNCNAME failed to hash the location URL (exit code: $exit_code)" >&2
+		return 4
 	fi
 
 	cached_header_file="$cache_dir/${hash}_headers"
@@ -206,14 +224,15 @@ conditional_get () {
 			/bin/cat "$cached_content_file"
 			return 0
 		fi
+		
 		conditional_get_mode=true
 	else
 		# ensure cache integrity
 		/bin/rm -f "$cached_header_file" "$cached_content_file" >&2
-		if $cache_only_mode; then
-			echo "ERROR: $FUNCNAME failed to find cached resource: $location" >&2
-			return 1
-		fi
+		
+		# quiet failure mode
+		$cache_only_mode && return 1
+		
 		conditional_get_mode=false
 	fi
 
@@ -260,20 +279,20 @@ conditional_get () {
 	do_conditional_get=false
 	if $conditional_get_mode; then
 		header_value=$( get_header_value "$cached_header_file" 'ETag' )
-		exit_code=$?
-		if [ $exit_code -ne 0 ]; then
-			echo "ERROR: $FUNCNAME: get_header_value failed to initialize" >&2
-			return $exit_code
+		return_code=$?
+		if [ $return_code -ne 0 ]; then
+			echo "ERROR: $FUNCNAME: get_header_value (return code: $return_code)" >&2
+			return 6
 		fi
 		if [ -n "$header_value" ]; then
 			do_conditional_get=true
 			curl_opts="${curl_opts} --header 'If-None-Match: $header_value'"
 		else
 			header_value=$( get_header_value "$cached_header_file" 'Last-Modified' )
-			exit_code=$?
-			if [ $exit_code -ne 0 ]; then
-				echo "ERROR: $FUNCNAME: get_header_value failed to initialize" >&2
-				return $exit_code
+			return_code=$?
+			if [ $return_code -ne 0 ]; then
+				echo "ERROR: $FUNCNAME: get_header_value (return code: $return_code)" >&2
+				return 6
 			fi
 			if [ -n "$header_value" ]; then
 				do_conditional_get=true
@@ -288,8 +307,8 @@ conditional_get () {
 	eval $cmd
 	exit_code=$?
 	if [ $exit_code -ne 0 ]; then
-		echo "ERROR: $FUNCNAME: curl failed ($exit_code)" >&2
-		return $exit_code
+		echo "ERROR: $FUNCNAME: curl failed (exit code: $exit_code)" >&2
+		return 5
 	fi
 
 	#######################################################################
@@ -304,14 +323,14 @@ conditional_get () {
 	# sanity check
 	if [ ! -f "$tmp_header_file" ]; then
 		echo "ERROR: $FUNCNAME unable to find header file $tmp_header_file" >&2
-		return 1
+		return 3
 	fi
 
 	response_code=$( get_response_code "$tmp_header_file" )
-	exit_code=$?
-	if [ $exit_code -ne 0 ]; then
-		echo "ERROR: $FUNCNAME: get_response_code failed to initialize" >&2
-		return $exit_code
+	return_code=$?
+	if [ $return_code -ne 0 ]; then
+		echo "ERROR: $FUNCNAME: get_response_code failed (return code: $return_code)" >&2
+		return 7
 	fi
 	$verbose_mode && printf "$FUNCNAME received response code: %d\n" "$response_code" >> "$tmp_log_file"
 
@@ -319,10 +338,10 @@ conditional_get () {
 
 		# sanity check
 		declared_content_length=$( get_header_value "$tmp_header_file" 'Content-Length' )
-		exit_code=$?
-		if [ $exit_code -ne 0 ]; then
-			echo "ERROR: $FUNCNAME: get_header_value failed to initialize" >&2
-			return $exit_code
+		return_code=$?
+		if [ $return_code -ne 0 ]; then
+			echo "ERROR: $FUNCNAME: get_header_value failed (return code: $return_code)" >&2
+			return 6
 		fi
 		actual_content_length=$( /bin/cat "$tmp_content_file" \
 			| /usr/bin/wc -c \
@@ -331,7 +350,7 @@ conditional_get () {
 		if [ -n "$declared_content_length" ]; then
 			if [ "$declared_content_length" != "$actual_content_length" ]; then
 				echo "ERROR: $FUNCNAME failed content length check" >&2
-				return 1
+				return 3
 			fi
 		else
 			echo "WARNING: Content-Length response header missing" >&2
@@ -351,22 +370,25 @@ conditional_get () {
 		exit_code=$?
 		if [ $exit_code -ne 0 ]; then
 			/bin/rm -f "$cached_header_file" "$cached_content_file" >&2
-			echo "ERROR: $FUNCNAME failed copy to file $cached_header_file" >&2
-			return $exit_code
+			echo "ERROR: $FUNCNAME failed copy to file $cached_header_file (exit code: $exit_code)" >&2
+			return 8
 		fi
 		/bin/cp -f "$tmp_content_file" "$cached_content_file" >&2
 		exit_code=$?
 		if [ $exit_code -ne 0 ]; then
 			/bin/rm -f "$cached_header_file" "$cached_content_file" >&2
-			echo "ERROR: $FUNCNAME failed copy to file $cached_content_file" >&2
-			return $exit_code
+			echo "ERROR: $FUNCNAME failed copy to file $cached_content_file (exit code: $exit_code)" >&2
+			return 8
 		fi
 
 	elif [ "$response_code" = "304" ]; then
+		# quiet failure mode
+		$force_output_mode && return 1
+		
 		$verbose_mode && echo "$FUNCNAME downloaded 0 bytes (cache is up-to-date)" >> "$tmp_log_file"
 	else
 		echo "ERROR: $FUNCNAME failed with HTTP response code $response_code" >&2
-		return 1
+		return 9
 	fi
 
 	#######################################################################
@@ -374,13 +396,8 @@ conditional_get () {
 	# output the resource content
 	#
 	#######################################################################
-
-	if $force_output_mode && [ "$response_code" = "304" ]; then
-		echo "ERROR: $FUNCNAME failed to get fresh resource: $location" >&2
-		return 1
-	fi
 	
-	/bin/cat "${cached_content_file}"
+	/bin/cat "$cached_content_file"
 	return 0
 }