Re: [whatwg/streams] Should cancelation error from UnderlyingSource be transferred too? (Issue #1259)

The short answer is that it's because transferable streams are implemented using a pipe, and pipes propagate cancellation without waiting.

Elaborating a bit, what we have is
```
original ---pipe---> writable <==CRIT==> transferred
```
(where "CRIT" means cross-realm identity transform). The internal pipe is invisible to developers, except to the extent that it dictates the semantics.

When `transferred` is cancelled, this is propagated back by erroring `writable`. Erroring is a synchronous operation, so there's no opportunity to wait.

When the pipe sees the `writable` is errored, it propagates it back by cancelling the `original` stream. Because `writable` is already errored, the results of cancellation cannot  be propagated back up the pipe.

See also https://github.com/whatwg/streams/issues/976.

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