RFR: 1581: Commit command error message misleading [v2]

[Zhao Song]

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

Zhao Song has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  SKARA-1581

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Changes:
  - all: https://git.openjdk.org/skara/pull/1405/files
  - new: https://git.openjdk.org/skara/pull/1405/files/1d87ca70..14bc1963

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1405&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1405&range=00-01

  Stats: 17 lines in 4 files changed: 4 ins; 8 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