RFR: 2189: Mark command as handled when JCheckConfiguration is missing or invalid in PullRequestCommandWorkItem

[Zhao Song]

Recently, we found that the skara bot keeps trying to process /open command in this pr https://github.com/openjdk/jdk8u-dev/pull/393

After investigation, we found that this pr is a dependent pr and this pr was closed before the parent pr(https://github.com/openjdk/jdk8u-dev/pull/392) got integrated, so this dependent pr didn't get retargeted.

Skara bot was trying to process the command but the bot couldn't find jcheck configuration in the target branch because the target is already deleted. And when the jcheck configuration is invalid or missing in PullRequestCommandWorkItem, RuntimeException would be thrown, causing the bot to repeatedly process the command. This behavior is wrong because when jcheck configuration is missing or invalid, it wouldn't recover automatically. In this case, the bot should just notice the user what problem it found and mark the command as handled to prevent the bot from being stuck in a loop.

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

Commit messages:
 - SKARA-2189

Changes: https://git.openjdk.org/skara/pull/1616/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1616&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2189
  Stats: 60 lines in 2 files changed: 56 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1616.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1616/head:pull/1616

PR: https://git.openjdk.org/skara/pull/1616