Fix a bug on windows when isHiddenFile returns an error

[anthonyraymond]

On windows when the isHiddenFile() methods returns an error it is of type os.Errno.

At another location on the code there is this particular piece of code:

list, err = w.list(name)
if err != nil {
	if os.IsNotExist(err) {
		w.mu.Unlock()
		if name == err.(*os.PathError).Path {
			w.Error <- ErrWatchedFileDeleted
			w.Remove(name)
		}
		w.mu.Lock()
	} else {
		w.Error <- err
	}
}

where list() uses isHiddenFile.

The problem is that os.IsNotExist(err) returns true for os.Errno but fails to cast to os.PathError which results in weird lock state because of the panic.