Call stopBeingLeader() if becomeLeader() fails

[kfaraz]

Description

If a master node fails to acquire leadership due to exception while calling listener.becomeLeader(),
listener.stopBeingLeader() must be called in order to ensure that there are no modules which
continue to believe that they are still the leader.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[kfaraz]

Thanks for the feedback, @gianm !
I have updated the patch accordingly.

[gianm]

LGTM. Coverage checker is unhappy; is it possible/useful to add coverage, or should we ignore it? Probably the System.exit(1) piece at least isn't testable in unit tests.

[kfaraz]

Thanks for the review, @gianm !
I will try to add some coverage for the changed lines today.
If it does not seem feasible (due to the system exit calls), then I will proceed with the merge.