in case of maintenance the error page returns http status 503.

This is necessary to enable the desktop sync client to react properly.
Currently the SabreDAV plugin OC_Connector_Sabre_MaintenancePlugin is not executed because this error page is returned before the SabreDAV code is executed
Merge pull request #1181 from owncloud/return-503-in-maintenance

in case of maintenance the error page returns http status 503.