LoginDialog: clear previous auth failed messages when trying again

When the user has the entered the password for the second time
and clicked OK, clear messages from the previous attempt, so any
new failure is shown clearly.

https://bugzilla.gnome.org/show_bug.cgi?id=687132

js/gdm/util.js

@@ -125,6 +125,9 @@ const ShellUserVerifier = new Lang.Class({
     },
 
     answerQuery: function(serviceName, answer) {
+        // Clear any previous message
+        this.emit('show-message', null, null);
+
         this._userVerifier.call_answer_query(serviceName, answer, this._cancellable, null);
     },