[WIP] Return XML server error when called through remote.php

:no_entry_sign: NOMERGE

Not fully tested, this is a tentative fix for #6119.
I noticed that the sync client doesn't show anything when getting those Sabre errors, possibly because we need to return an HTTP code 500 first...

Added OC::$ISREMOTE to tell base.php to not output errors to stdout but
rather throw an exception. That exception is caught by remote.php and
converted to an XML response