[FLINK-19012][task] Check AsyncCheckpointRunnable status before throwing an exception

[rkhachatryan]

What is the purpose of the change

Currently, AsyncCheckpointRunnable throws an exception if SubtaskCheckpointCoordinatorImpl is closed.
However, it should also check its own status as it might be a normal case.

Verifying this change

The change is covered by existing end-to-end tests which are currently failing.
Unit testing would involve concurrency which I think would be overkill for essentially a logging problem.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? no

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Last check on commit 9d1f76c (Thu Aug 27 18:34:54 UTC 2020)

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[AHeise]

Changes LGTM. Could we add a unit test to cover this change?

[rkhachatryan]

I don't see any easy way to test it because there are 2 threads involved and SubtaskCheckpointCoordinatorImpl and AsyncCheckpointRunnable being tightly coupled.
Given that the only issue impact is CI failures, I think we can skip testing this fix.

[AHeise]

Okay, I had hoped it is easier, but your arguments are convincing that it's not. LGTM.

[rkhachatryan]

Thanks for reviewing and merging!