RFR: 1616: Improve error message when an author isn't found in GitLab [v2]

[Guoxiong Li]

On Wed, 5 Oct 2022 18:04:01 GMT, Zhao Song <zsong at openjdk.org> wrote:

>> Improved the error message in GitLabMergeRequest:author
>
> Zhao Song has updated the pull request incrementally with one additional commit since the last revision:
> 
>   make the error message more detailed

Marked as reviewed by gli (Committer).

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PR: https://git.openjdk.org/skara/pull/1390