RFR: 1070: An invalid /backport command target causes bot to fail endlessly

[Erik Joelsson]

If a user issues a /backport command on a commit on a repository that exists, but for which we haven't properly configured backports to work (they need a fork defined where the PR can be created from), we currently end up with endless failures. This patch adds handling of the situation by printing a helpful message for the user, which also includes a list of all valid repositories.

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

Commit messages:
 - SKARA-1070

Changes: https://git.openjdk.java.net/skara/pull/1182/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1182&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1070
  Stats: 65 lines in 3 files changed: 60 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/1182.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1182/head:pull/1182

PR: https://git.openjdk.java.net/skara/pull/1182