Improve error message if task fails to return start time

[kfaraz]

Description

While running the K8s task runner, tasks sometimes fail with the error message:

Task[xyz] failed to return start time, killing task.

The Overlord logs contain slightly more info

Task[xyz] failed to return start time, killing task. Service [xyz] handler closed unexpectedly.

Changes

• Improve task failure error message. New message would be:

Failed to return start time: Service [xyz] handler closed unexpectedly.

• Add log line in ServiceClientImpl to identify root cause in future failures.

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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 reviews, @cryptoe , @abhishekagarwal87 !