Fix Task Canceling

[magniloquency]

This PR closes #242.

Root cause: VanillaWorkerController.on_task_cancel() didn't conform to its interface, because it did not return the worker ID. It also sent out the task cancel message, which the caller (VanillaTaskController.__send_task_cancel_to_worker()) was also set up to do, assuming the worker id was returned.

Solution: Ensure that the worker id is always returned and check for validity in the caller. Only actually send the cancel message in the vanilla task controller.

Recommendation: Python 3.12 introduced typing.override, which is available in pre-3.12 versions using typing_extensions, and enables type checking of overridden methods. Mypy also has an option to force all overridden methods to use the @override decorator. Mypy won't warn us for methods with a dynamic return type (no return type hint), but it might be possible for it to warn us on that as well.

[sharpener6]

@magniloquency, can you remind me why you closed this one?

[magniloquency]

@sharpener6, I closed this because, if I remember correctly, this PR fixed one problem but exposed another that would need fixes in the state machines. You said you would take a look at it, so I closed this PR thinking that you would raise your own