[FLINK-13124] Don't forward exceptions when finishing SourceStreamTask

[aljoscha]

What is the purpose of the change

Before, exceptions that occurred after cancelling a source (as the
KafkaConsumer did, for example) would make a job fail when attempting a
"stop-with-savepoint". Now we ignore those exceptions.

Brief change log

• add a isFinished flag in SourceStreamTask
• check this flag before re-throwing exceptions

Verifying this change

This change added tests and can be verified as follows:

• Added unit tests in SourceStreamTaskTest

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

• Checkpointing, yes, because it fixes stop-with-savepoint

Documentation

• Does this pull request introduce a new feature? no

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Automated Checks

Last check on commit a7f8c3e (Tue Aug 06 15:42:49 UTC 2019)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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[kl0u]

Merging this on the 1.9

[flinkbot]

CI report:

• a7f8c3e : SUCCESS Build

[kl0u]

Merge on 1.9.