RFR: pr-bot: throw on unexpected jcheck issues

[Erik Helin]

Hi all,

please _carefully_ review this patch that throws `IllegalStateException` for
some jcheck issues that shouldn't happen for commits that the PR bot has
created. I think I have gone through all cases, but would appreciate careful
reviewing here ��

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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

Commits:
 - b3e60e05: pr-bot: throw on unexpected jcheck issues

Changes: https://git.openjdk.java.net/skara/pull/423/files
 Webrev: https://webrevs.openjdk.java.net/skara/423/webrev.00
  Stats: 9 lines in 1 file changed: 2 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/423.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/423/head:pull/423

PR: https://git.openjdk.java.net/skara/pull/423