Use common error when removing an unwatched file (#460)

Use common error when removing an unwatched file (#460)

The errors returned by the various implementations of the watcher are all different
which makes handling them difficult. This PR follows the suggestion in:
https://github.com/fsnotify/fsnotify/pull/455#issuecomment-1146037157 by @mattn
to create a common error which is wrapped by the implementations.

Replaces: https://github.com/fsnotify/fsnotify/pull/455
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Andrew Thornton <art27@cantab.net>