// Is a one place function which converts all os.PathError
// into a more FS object layer friendly form, converts
// known errors into their typed form for top level
// interpretation.
func osErrToFileErr(err error) error {
	if err == nil {
		return nil
	}
	if osIsNotExist(err) {
		return errFileNotFound
	}
	if osIsPermission(err) {
		return errFileAccessDenied
	}