Add error event when negotiation fails

When negotiation fails, it fails silently, neither closed or error is
emitted on the connection. Thus, there is no way to know if the
connection negotiation failed.

lib/negotiator.js

@@ -146,8 +146,12 @@ Negotiator._setupListeners = function(connection, pc, pc_id) {
 
   pc.oniceconnectionstatechange = function() {
     switch (pc.iceConnectionState) {
-      case 'disconnected':
       case 'failed':
+        util.log('iceConnectionState is disconnected, closing connections to ' + peerId);
+        connection.emit('error', new Error('Negotiation of connection to ' + peerId + ' failed.'));
+        connection.close();
+        break;
+      case 'disconnected':
         util.log('iceConnectionState is disconnected, closing connections to ' + peerId);
         connection.close();
         break;