Allow checking when no codeowners file is found

[pablochacin]

This will allow checking when no codeowners exists:

fsys := os.DirFS(cwd())

c, err  := FromFileWithFS(fsys, ".")

if errros.Is(err, ErrNoCodeownersFound) {
  // handle this case
}

[hairyhenderson]

Thanks, this is great!

[hairyhenderson]

@pablochacin could I get you to amend your commit with a message like "feat(errors): Allow checking when no codeowners file is found"? Then this can merge.