RFR: 396: Add some additional help for empty PRs

[Robin Westberg]

Hi all,

Please review this change that adds a placeholder message that can help the user to determine how exactly the "empty PR
body" error should be remedied.

Best regards,
Robin

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Commit messages:
 - Add a replace-me marker

Changes: https://git.openjdk.java.net/skara/pull/621/files
 Webrev: https://webrevs.openjdk.java.net/skara/621/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-396
  Stats: 18 lines in 2 files changed: 12 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/621.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/621/head:pull/621

PR: https://git.openjdk.java.net/skara/pull/621