[streams] Uses of "upon fulfillment/rejection" contain unused returns and throws (#284)

Several algorithms in the Steams spec use the [Upon fulfillment](http://www.w3.org/2001/tag/doc/promises-guide#upon-fulfillment) and [Upon rejection](http://www.w3.org/2001/tag/doc/promises-guide#upon-rejection) algorithms.

Taking [WritableStreamAdvanceQueue()](https://streams.spec.whatwg.org/#writable-stream-advance-queue) as an example, the spec has:

1. ...
1. Let writeResult be ...
1. Upon fulfillment of writeResult,
  1. ...
  1. If ..., then call-with-rethrow Call(stream@[[error]], undefined, «‍syncResult.[[value]]»).
  1. Otherwise, return WritableStreamAdvanceQueue(stream).
1. Upon rejection of writeResult with reason r, call-with-rethrow Call(stream@[[error]], undefined, «‍r»).

"Upon fulfillment" is defined as calling p.then(_nested steps_), but not as doing anything with the return value of `.then()`. So any exceptions thrown or values returned by these nested steps are simply ignored, and the final "return" or "call-with-rethrow" could be removed.

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/284