Add HttpListenerException processing in case of access denied

OAuthDesktopApp/OAuthDesktopApp/MainWindow.xaml.cs

@@ -69,8 +69,22 @@ private async void button_Click(object sender, RoutedEventArgs e)
             var http = new HttpListener();
             http.Prefixes.Add(redirectURI);
             output("Listening..");
-            http.Start();
-
+
+            try
+            {
+                http.Start();
+            }
+            catch (HttpListenerException ex)
+            {
+                if (ex.HResult == unchecked((int) 0x80004005))
+                {
+                    MessageBox.Show("Access denied. Try running the program as an administrator.", "OAuthDesktopApp",
+                        MessageBoxButton.OK, MessageBoxImage.Warning);
+
+                    return;
+                }
+            }
+
             // Creates the OAuth 2.0 authorization request.
             string authorizationRequest = string.Format("{0}?response_type=code&scope=openid%20profile&redirect_uri={1}&client_id={2}&state={3}&code_challenge={4}&code_challenge_method={5}",
                 authorizationEndpoint,