Make it easier to debug path parsing failures

I have a non-trivial bit of code using `rustix` that is returning `EINVAL`. I thought it was an OS error, but it turns out that I was getting https://github.com/rust-lang/rust/issues/116523. However it was being masked by the fact that `Arg` impl returns `EINVAL` if there is a `\0` in the path bytes. It would be nice if `rustix` returned an error code less plausible than `EINVAL` when a path failure like this happens.