Parse error messages

Error messages can contain wikitext, so should be parsed.
E.g. https://www.mediawiki.org/wiki/MediaWiki:Abusefilter-disallowed
Without parsing the error message, this results in:
[[Image:Stop x nuvola.svg|50px|link=]] This action has been automatically identified as harmful, and has been '''disallowed.''' If you believe your edit was constructive, please inform an [[Project:Administrators|administrator]] of what you were trying to do. A brief description of the abuse rule which your action matched is:

Change-Id: If8fa1fdb35a4b99e1ed60985b4318ffc9bedb5f9