Fix invalid call to defer.errback.
Fix invalid call to defer.errback.

Replaced defer.errback with defer.fail. defer.fail is called in
the defer module, defer.errback is called on Deferred instances,
of which none was available in this case.

driftx/Telephus,ClearwaterCore/Telephus,driftx/Telephus,ClearwaterCore/Telephus,Metaswitch/Telephus,Metaswitch/Telephus