State method ``.into_file`` has difficulty if current instantiated states is smaller that in the existing file

[JoelPasvolsky]

If you save two states into a file (model.states.to_file()) you can replace those two states with new ones and also add states (model.states.into_file()) but if you try writing fewer states (either with smaller model.states.size() or with an uninitialized state), you get an error. Functionally that's fine but the error is unclear (BadZipFile: Bad magic number for central directory) so it would be nice to catch this with a clearer error.