[FLINK-30538][SQL gateway/client] Improve error handling of stop job operation

[link3280]

What is the purpose of the change

Currently, the stop-job operation produces some verbose error msg and doesn't handle exceptions in stop-without-savepoint gracefully.

This PR fixes the problem.

Brief change log

• Wrap simple cancel with try-catch.
• Wait for simple cancel Acknowledge before returning 'OK'.
• Simplify exception message for stop job operations.

Verifying this change

Please make sure both new and modified tests in this PR follows the conventions defined in our code quality guide: https://flink.apache.org/contributing/code-style-and-quality-common.html#testing

This change is a trivial rework / code cleanup without any test coverage.

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, 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? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 633fa1a Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[link3280]

@flinkbot run azure

[link3280]

Please kindly take a look @fsk119

[fsk119]

Thanks for your update.

[link3280]

CI ran into https://issues.apache.org/jira/browse/FLINK-30328. Re-run CI.

[link3280]

@flinkbot run azure

[link3280]

@fsk119 CI turned green. Please kindly take a look at your convenience.

[link3280]

ping @fsk119 . It should be a quick one :)

[link3280]

ping @fsk119

[fsk119]

LGTM