return a 400 in authorizationService if client does not supply valid XML

[alaniwi]

If the client does not supply a valid XML document, currently the authorizationService returns a 500 with a stack trace.

The issue can be simply reproduced by doing a GET instead of a POST.

The org.opensaml.xml.parse.XMLParserException reported in the traceback should be caught, and a 400 returned.