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/Input.cpp

@@ -1727,6 +1727,11 @@ static void menuactionpress(void)
         game.returnmenu();
         map.nexttowercolour();
         break;
+    case Menu::errorloadinglevel:
+        music.playef(11);
+        game.returnmenu();
+        map.nexttowercolour();
+        break;
     default:
         break;
     }