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
what i mean is: if we have check on login true, we already did the sanity check, don't need to repeat it.

if it was disabled, we should do it now, to identify the real issue. but we should do the login check, not simply a GET on whatever url we happened to try. also if i try to get /cmf from a misconfigured server, this logic would not trigger now, but it should.