Do not close game if custom level has assets issues

It's quite rude to close the game entirely if there is trouble with
assets. Instead, just unload the assets and gracefully return to the
title screen.

desktop_version/src/Game.cpp

@@ -6477,6 +6477,9 @@ void Game::createmenu( enum Menu::MenuName t, bool samemenu/*= false*/ )
         option("silence");
         menuyoff = 10;
         break;
+    case Menu::errorloadinglevel:
+        option("ok");
+        break;
     }
 
     // Automatically center the menu. We must check the width of the menu with the initial horizontal spacing.