Make `ErrorKind::PathNotFound` consistent on inotify

[y5c4l3]

Before this patch, watching a nonexistent path using inotify backend would give an ErrorKind::Io variant. This behavior was inconsistent with other backends.

I am not sure if this is expected but taking #110 into consideration I think the path error ought to be consistent.

Run the inotify watcher on Linux:

let error = watcher.watch(Path::new("/nonexistent"), notify::RecursiveMode::Recursive).unwrap_err();

Before:

Error { kind: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" }), paths: [] }

After:

Error { kind: PathNotFound, paths: [] }

[0xpr03]

Thanks!