Fix event name

We successfully retry after a websocket connection closes, but this typo
prevented us from retrying if the websocket simply never opened.

// FREEBIE

libtextsecure/message_receiver.js

@@ -58,7 +58,7 @@
             TextSecureServer.getDevices(this.number).
                 then(this.connect.bind(this)). // No HTTP error? Reconnect
                 catch(function(e) {
-                    var ev = new Event('textsecure:error');
+                    var ev = new Event('error');
                     ev.error = e;
                     eventTarget.dispatchEvent(ev);
                 });