Properly forward cancellation reason

[fjetter]

This will attach a reason to the exception if we know the cause. The causes spelled out here are still rather ambiguous but it's a little better than just "unknown"

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    27 files  ±0      27 suites  ±0   11h 49m 14s ⏱️ + 10m 57s
 4 103 tests ±0   3 988 ✅  - 2    111 💤 ±0  4 ❌ +2 
51 441 runs  ±0  49 140 ✅  - 2  2 297 💤 ±0  4 ❌ +2 

For more details on these failures, see this check.

Results for commit 2bd2c0e. ± Comparison against base commit e931ccd.

♻️ This comment has been updated with latest results.

[fcourtial]

@fjetter Could we release a version with this fix?

I was about to contribute the same kind of fix, when we cancel a Future on purpose, for instance because the user wants to stop the running job, then a FutureCancelledException is triggered with the reason unknown whereas we did provide a reason when cancelling.