[streams] Throw on underlying source double-close or double-error (#303)

Fixes #298.

Quick one to review @tyoshino 
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/303

-- Commit Summary --

  * Throw on underlying source double-close or double-error

-- File Changes --

    M index.bs (28)
    M reference-implementation/lib/readable-stream.js (24)
    M reference-implementation/test/bad-underlying-sources.js (99)
    M reference-implementation/test/readable-stream.js (51)

-- Patch Links --

https://github.com/whatwg/streams/pull/303.patch
https://github.com/whatwg/streams/pull/303.diff

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https://github.com/whatwg/streams/pull/303