Let server know that we failed to process message on cache error

libtextsecure/message_receiver.js

@@ -180,6 +180,7 @@ MessageReceiver.prototype.extend({
             this.queueEnvelope(envelope);
           },
           error => {
+            request.respond(500, 'Failed to cache message');
             window.log.error(
               'handleRequest error trying to add message to cache:',
               error && error.stack ? error.stack : error