[New module system] editing interface files causes a panic

[yav]

If we try to load an interface file, Cryptol correctly says that it can't load the file. However, we do set the current module as the interface file, so if we do :edit we can edit the file. Exiting the editor tries to reload the file, and again fails (correctly), but then the REPL is left in an inconsistent state where trying to evaluate anything panic because the focused module is not loaded.