Trying to export unknown game gives stack trace

[perwestling]

After installing latest version, I go to 18xx-maker/games

If I do e.g.
18xx-export-rb game 18SJ
it works fine.

But if I have incorrect name of game, eg
18xx-export-rb games 18sj
I get the text "undefined" followed by
TypeError: Cannot read property 'file' of undefined

I suggest adding a check if game exists, and if not showing some suitable error message.