RFR: contributor command: friendlier message when removing contributor

[Erik Helin]

Hi all,

please review this patch that prints a bit friendlier message when removing a
contributors. The message will now include the contributors associated with the
pull request _if_ the given contributor is not one of them.

Testing:
- added a new unit test for the feature
- `make test` passes on Linux x64

Thanks,
Erik

-------------

Commits:
 - 89621068: contributor command: friendlier message when removing contributor

Changes: https://git.openjdk.java.net/skara/pull/425/files
 Webrev: https://webrevs.openjdk.java.net/skara/425/webrev.00
  Stats: 58 lines in 2 files changed: 56 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/425.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/425/head:pull/425

PR: https://git.openjdk.java.net/skara/pull/425