Replace Non-Exhaustive Pattern with Case

I did this because in newer versions of GHC things like this require a
MonadFail instance, whereas explicitly handling it like this does not.

Now, we don't currently require those newer versions, but since I happened to
have a new one it was causing me trouble. And this new way works in both
versions, so I think it's safe to just throw it in here.

Current version in use by cheogram

Initial commit