RFR: Improve error message for non-authors creating Merge PRs

[Robin Westberg]

Hi all,

Please review this small change that improves the error message when a non-author tries to create a Merge PR.

Best regards,
Robin

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Commit messages:
 - Improve error message

Changes: https://git.openjdk.java.net/skara/pull/608/files
 Webrev: https://webrevs.openjdk.java.net/skara/608/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/608.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/608/head:pull/608

PR: https://git.openjdk.java.net/skara/pull/608