gitlab migration: fals token error is not reported

[6543]

gitea v: 1.13.0+dev-583-g090024db3

Migrate repository from https://gitlab.com/gitea/test_repo failed: error: GitlabDownloader is nil

[jolheiser]

Doing a little testing on try, it seems to work with a token. But migrating fails if using GitLab without a token.

[jolheiser]

I think what needs to happen here is that if no auth is passed, we should use the plain git downloader because the GitLab client expects to log into GL.

[6543]

@jolheiser the first api call retuned this: {message: 401 Unauthorized} - if token is wrong/outdated ->
currently it only will thell Downloader == nil but not the reason

[jolheiser]

Gotcha. Alright, so then it needs a small refactor as well.

[6543]

also notized we need a collor-code noramlizer: ( gitlab allow code #fde -> #ffddee )