RFR: Add optional handler for workitem runtime errors

[Robin Westberg via github.com]

Hi all,

Please review this change that allows `WorkItem`s that use the PullRequestUpdateCache to invalidate an entry if an RuntimeException occurs during the execution. Without this, operations are not retried automatically on transient errors.

Best regards,
Robin

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Commits:
 - e5ea9a54:	Add optional handler for workitem runtime errors

Pull request:
https://git.openjdk.java.net/skara/pull/100

Webrev:
https://webrevs.openjdk.java.net/skara/100/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/100.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/100/head:pull/100