Suppress stack traceback on invalid XML

[iay]

Migrated from GitLab issue 3:

The java-support library's default configuration of BasicParserPool gives a stack trace if the source can't be parsed. This really needs to be fixed upstream, but we can suppress it by setting a different ErrorHandler with more gentle behaviour in our code.

[iay]

Upstream ticket is JSPT-81: Default logging setup in BasicParserPool considered harmful.

This has been resolved for java-support v7.4.0, which will be finalised for IdP v3.4.0. If we were really fussed, we could pull in the SNAPSHOT version of this library, so I don't think we really need to address this independently. Will leave this open, though, until it's resolved one way or another.