Allow propagating errors from Asyncified imports back to C++

[RReverser]

Right now, from what I understand, there is no way to wake up the C++ side with an error instead of a successful value. This makes errors in Asyncified functions behave differently than in regular imports in that, upon an async error, the C++ code can never be resumed, and such error has no chance of getting handled, and instead only ends up being printed to the Console.

This is not very important for wakeUp based handler, but in promises rejections are fairly common, and would be good to propagate those somehow back to C++.

I'd imagine following Promise's design here might be easy enough; basically, they have a resolve and reject callbacks, and are guaranteed that whatever is called first, wins, and no further resolution/rejection occurs.

As far as I can tell, wakeUp is a direct analogy for the resolve callback, so I'd imagine we could simply extend API to accept a reject callback as well, store it on the state, and on C++ side it would call throw instead of continuing execution.

[kripken]

Sounds reasonable to me. Is there a risk of breaking existing users by adding it?

[RReverser]

There is a potential risk that silently swallowed errors will suddenly be thrown on C++ side, but I suppose

1. It's a better state of things anyway as it only reveals already existing problems and makes them easier to catch.
2. Worst case, that error should get propagated back from C++ to JS again and still printed to console, but with more informative stacktrace.

So I don't see how this would break any valid real-world code.

[stale]

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