[streams] TransformStream: There's no means available to fail transform() asynchronously (#331)

An exception thrown by `transform()` is caught to error both `readable` and `writable`. But if `transform()` wants to do something asynchronous that may fail, we cannot tell the TransformStream the failure. `transformDone` just tells it to retrieve the next chunk.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/331