[whatwg/streams] `read()` doesn't reject if queue is non-empty and pull() throws (#667)

I found this behaviour surprising:

```javascript
promise_test(t => {

  let pullShouldThrow = false;
  const rs = new ReadableStream({
    pull(controller) {
      if (pullShouldThrow) {
        throw error1;
      }
      controller.enqueue(0);
    }
  }, new CountQueuingStrategy({highWaterMark: 1}));
  const reader = rs.getReader();
  return Promise.resolve().then(() => {
    pullShouldThrow = true;
    return Promise.all([
      reader.read(),
      promise_rejects(t, error1, reader.closed, '.closed promise should reject')
    ]);
  });

}, 'read should not error if it dequeues and pull() throws');
```

`read()` doesn't reject because it enters step two at https://streams.spec.whatwg.org/#rs-default-controller-private-pull and so returns before the failure from `pull()` is asynchronously detected.

If you change highWaterMark to 0 then you get the other code path and `read()` rejects.

Having thought about it a bit this is probably correct.

I couldn't find a test for this. If everyone agrees it's the correct behaviour and no-one recalls seeing a test for it anywhere I will contribute this one to w-p-t.

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