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
should we not try to GET the root node? if the POST was to some path deep down somewhere, or a resource that does not have a GET, the GET on that would fail as well and we would produce a misleading message.

what if we would do the check login on server thing? https://github.com/jackalope/jackalope-jackrabbit/blob/improve_error_handling/src/Jackalope/Transport/Jackrabbit/Client.php#L355-L357 - the only situation when invalid configuration is the cause for an error is when the check was disabled. we could extract the check login method and call it from here.