In case of a 500 error, attempt a GET to determine if the URI might be incorrect

fix for https://github.com/doctrine/DoctrinePHPCRBundle/issues/154
i originally had the handling only in the section for checking on login but figured it would make sense to always check for this .. then again if the login worked, then everything else should work from there on too. so maybe its indeed enough to just check in the case of check on login.