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

[Zhao Song]

Improved the error message in GitLabMergeRequest:author

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Commit messages:
 - Improved error message in GitLabMergeRequest::author

Changes: https://git.openjdk.org/skara/pull/1390/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1390&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1616
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1390.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1390/head:pull/1390

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