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What's changed and what's your intention?

Previously in #16816, we used a try_join_all directly on the JoinHandle, which only returns the JoinError, while the error in the output of JoinHandle is ignored, because it is treated as Ok(Err(...)).

In this PR, we transform the future of JoinHandle to a single layer result, so that the error in join handle output will be treated as error.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
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Documentation

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Release note

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