Default file to run
Right now if you don't pass any arguments to pyg.exe, it simply displays usage information in the terminal and exits. We can do better than this.
First, we can make it attempt to run a hardcoded file name, for example game.zip (and maybe also fall back to running game/main.py) -- so that people don't need to add a .bat file to their distributions.
Second, if the files are not found, we can make it display an actual window with explanations -- Windows users are unlikely to read console messages, for some reason.