enhance validateable interface (#7714)

This PR is part of https://codeberg.org/forgejo/forgejo/pulls/4767 filed by @algernon

Validateable is enhanced for less duplication.

## Checklist

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### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Co-authored-by: zam <mirco.zachmann@meissa.de>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7714
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-committed-by: Michael Jerger <michael.jerger@meissa-gmbh.de>

modules/forgefed/actor_test.go

@@ -145,7 +145,7 @@ func TestPersonIdValidation(t *testing.T) {
 	sut.HostPort = 443
 	sut.IsPortSupplemented = true
 	sut.UnvalidatedInput = "https://an.other.host/api/v1/activitypub/user-id/1"
-	if sut.Validate()[0] != "Value forgejox is not contained in allowed values [forgejo gitea]" {
+	if sut.Validate()[0] != "Field Source contains the value forgejox, which is not in allowed subset [forgejo gitea]" {
 		t.Errorf("validation error expected but was: %v\n", sut.Validate()[0])
 	}
 }