Bug introduced by #7998
### Tests
- go to your settings page https://v12.next.forgejo.org/user/settings
- try to change to an invalid username (like `.well--invalid`)
- verify that no 500 error is shown, only a flash message
I tried to add a test in `tests/integration/user_test.go`, but failed to catch the error message:
```
resp := session.MakeRequest(t, req, http.StatusOK)
txt := resp.Body.String()
t.Log(txt) // no template error??
t.FailNow()
```
### Release notes
- [x] I do not want this change to show in the release notes.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8236
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>
285f66b782