added error handling when requesting to join non existing room

public/static/js/game.js

@@ -410,6 +410,10 @@
                     MapGuesser.showModalWithContent('Error', 'This game is already started, you cannot join.');
                     break;
 
+                case 'game_not_found':
+                    MapGuesser.showModalWithContent('Error', 'The game room was not found by this ID. Please check the link.');
+                    break;
+
                 default:
                     MapGuesser.showModalWithContent('Error', 'Error code: \'' + error + '\'');
                     break