Implement Channel.prototype.throw

[brainkim]

The AsyncIterator defines an optional throw method that can be used to make the iterator throw an error. This is useful when you have nested async iteration loops and you don’t want to wait for the child loop to terminate before continuing the parent loop, but you still want to propagate errors to avoid Promise rejections. Example:

for await (const a of iterator) {
  if (a.subscribe) {
    (async () => {
      for await (const b of a.subscribe()) {
        console.log(b);
      }
    // propagate errors to the parent
    })().catch((err) => iterator.throw(err));
  }
}

Figure out the rejection semantics for throw and how it interacts with next and return. My plan is for throw to close the channel with the error and await return. If the channel is already closed, should throw reject?