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

[Zhao Song]

> 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

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Changes:
  - all: https://git.openjdk.org/skara/pull/1390/files
  - new: https://git.openjdk.org/skara/pull/1390/files/cbb90d81..6c0c2fd7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1390&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1390&range=00-01

  Stats: 1 line in 1 file changed: 0 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