Propagate BaseException raised in pool workers

[chjnett]

Closes #427.

A task that raises a BaseException (plain BaseException, SystemExit,
KeyboardInterrupt, …) was not caught by the worker's result marshalling —
the worker only caught Exception — so the exception propagated out of the
worker, the worker exited prematurely, and the caller got a confusing
WorkerLostError instead of the exception.

Change:

• billiard/pool.py: the worker's task-execution catch is now except BaseException, so any exception a task raises is marshalled back as a
  result and re-raised by ApplyResult.get() instead of killing the worker.

Test:

• Added test_base_exception_propagates, which submits a task raising
  BaseException and asserts result.get() raises it.

Ran pytest t/unit/test_pool.py locally — all 7 tests pass (including the
new one); without the fix the new test fails. The ExceptionInfo machinery
already handles any exception type (it rebuilds the original exception when
unpickling), so no further changes were needed on the parent side.

[Copilot]

Pull request overview

This PR fixes Pool worker behavior so that tasks raising BaseException (e.g., BaseException, SystemExit, KeyboardInterrupt) are marshalled back to the parent process and re-raised by ApplyResult.get(), instead of causing the worker to exit and the parent to raise a misleading WorkerLostError (closes #427).

Changes:

• Catch BaseException (not just Exception) around task execution in pool workers so failures are returned as ExceptionInfo.
• Add a unit test asserting that a BaseException raised in a worker is re-raised by result.get().
• Document the behavioral change in CHANGES.txt.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
billiard/pool.py	Catch BaseException in the worker task execution path so exceptions are marshalled instead of killing the worker.
t/unit/test_pool.py	Adds a regression test to ensure BaseException propagates to the caller via ApplyResult.get().
CHANGES.txt	Notes the unreleased behavior change and references the fixing issue.

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