For two reasons:
- We'll later want apfl code to load other apfl code. If an IO error
happens in that case, we don't want that to be a fatal error, only a
regular error that can be catched in apfl (once we have something like
`try`).
- I want to get rid of fatal errors as a generic category completely.
Instead the error reporting functions themselves should tell you the type
of error directly, if it was something fatal.
