[streams] Make sure promise returning methods don't throw (#342)

I see some call-with-rethrows in promise returning methods' algorithm. Check that they're correctly implemented so that we don't throw but return a rejected promise to conform to http://www.w3.org/2001/tag/doc/promises-guide#always-return-promises.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/342