Added a check to test whether the empty-game.zip file exists

This one tripped me up when using it. I didn't realise that I needed to move the empty-game.zip to /usr/bin as well so I ended up getting an unreadable error message. This just creates a cleaner error message. 

There were a few other error checking measures that I wanted to implement but file IO (and even attribute checking) is rather platform specific so I didn't really feel comfortable implementing them in a cross platform piece of software.