Implemented unhandled error reporting for orphan fibers

[djspiewak]

This enriches CallbackStack with the ability to report when no callbacks were invoked. Note that this isn't entirely perfect, since a fiber which joins and is later canceled within the join will empty out its callback, but the joined fiber may finish with an error at the exact same moment. If this happens and the joining fiber is the only fiber, the error would have been side-channeled but might end up being missed. I think this is a fairly reasonable tradeoff though, at least for the moment. The alternative is that we force a memory barrier in this case, which I think is a bit too much deoptimization for this edge case.

Closes #2842

[djspiewak]

Test failures here are legit. I think it's a race condition in how I setup the test which is being surfaced randomly by the TestContext shuffling.

[RobertoUa]

Is it possible to override reportFailure in the default ctx? For instance, I want to report an exception to Sentry.

[armanbilge]

@RobertoUa I think that's what #2869 is doing, which will land in 3.4.0.