`--fs` accepts files, not just folders; errors follow

[aliceisjustplaying]

--fs correctly throws an error if a directory or a file doesn't exist, however TIC-80 is more than happy to start anyways with a file as an argument. Once started, save (predictably) fails.

Present in latest upstream as of commit https://github.com/nesbox/TIC-80/commit/bf5b95cf31bc40cc85ab400d723f95cb841436bb.