fixes igniterealtime#48: Remove spinner when chat request fails

When an end-user requests a chat through the embedded Converse client, a spinner starts running that ends when the chat starts.

When all agents reject the request, the user leaves the queue. Before this commit, the end-user was not visually informed of this (the spinner kept on spinning). This commit replaces the spinner with the chat button again (allowing for a retry).

There is probably a lot to improve on this, but that's beyond my capabilities with Converse.

classes/ofmeet.js

@@ -139,6 +139,21 @@ var ofmeet = (function(of)
             console.error('join workgroup error', err);
             document.getElementById("chatloader").style.visibility = "hidden";
         });
+
+        _converse.connection.addHandler(function(message) {
+            if (message.querySelector('depart-queue')) {
+                console.info("joining workgroup failed - departed the queue")
+                document.getElementById("chatloader").style.visibility = "hidden";
+                document.getElementById("chatbutton").style.visibility = "visible";
+                // TODO: give some kind of feedback to the end-user, explaining why this has happened.
+
+                // Got a 'depart-queue' message: remove this handler by returning 'false'.
+                return false; // TODO this handler should also be removed after successfully joining a room.
+            } else {
+                // Did not get a 'depart-queue' message: retain this handler by returning 'true'.
+                return true
+            }
+        }, 'http://jabber.org/protocol/workgroup','message');
     }
 
     function getFastPathStatus(callback)