[streams] Notate throwing vs. non-throwing abstract operations (#386)

Fixes #371.

@tyoshino, pretty sure this is correct, but if you could do a quick check to make sure my throw/nothrow annotations are OK, that'd be helpful.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/386

-- Commit Summary --

  * Notate throwing vs. non-throwing abstract operations

-- File Changes --

    M ecmarkupify.js (23)
    M index.bs (103)

-- Patch Links --

https://github.com/whatwg/streams/pull/386.patch
https://github.com/whatwg/streams/pull/386.diff

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https://github.com/whatwg/streams/pull/386