RFR: 1581: Commit command error message misleading

[Zhao Song]

This patch updated the warning message when a user is not authorized to use the command `/backport`, `/tag`, `/clean`.

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Commit messages:
 - SKARA-1581

Changes: https://git.openjdk.org/skara/pull/1405/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1405&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1581
  Stats: 15 lines in 4 files changed: 10 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1405.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1405/head:pull/1405

PR: https://git.openjdk.org/skara/pull/1405