Re: [streams] Do writable streams need separate signals for write-success vs. write-accepted? (#316)

One issue I just thought of is about error signaling. The separate-signals model gives you more chances to communicate errors. E.g., if you require all data written to the stream to be validated in a certain way, and that validation is async, the separate-signals model lets you communicate that with (1). Whereas if we only gave backpressure signals via the promise, then the promise would fulfill immediately if the queue was empty, and you wouldn't be able to know that your data failed validation except by listening to `ws.closed` or similar.

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https://github.com/whatwg/streams/issues/316#issuecomment-91041716