Small improvement of promise iteration error handling

[ehmicky]

This is a tiny improvement of some logic related to handling errors thrown in a for await (const line of nanoSpawn(...)) { ... } loop.

[ehmicky]

iterator is the lineIterator created for either stdout or stderr. This function combines both. If an error is thrown in the for loop, iterator.throw() for both stdout and stderr is called, then the original error is propagated.

The code works for any type of iterator. But in practice, we know that the iterator is always created by a generator function * () {}. That type of iterator:

• Always has a return() and throw() functions (which are otherwise optional for other iterators)
• Always has a throw() that re-throws the error passed as argument. Therefore, throw error is never reached.

So this PR does a small adjustment to remove a few bytes, by optimizing the fact that we know that iterator is coming from a generator function.