Pull Request for Adding registration error callback for Android

If there is an error registering with GCM on Android, the error code is
propogated to ecb in javascript.

On Android, if there is an error registering for Push Notifications, onError() and onRecoverableError() handlers of GCMIntentService are invoked and error ID is passed to them. The error ID should be passed back to Javascript event callback (ecb) with the following JSON:
{
  "event":"error",
  "error":errorID
}