[whatwg/fetch] Make extract throw for a disturbed/locked ReadableStream (#801)

This also introduces "safely extract" which mainly serves to make it explicit which extraction operations cannot throw.

Tests: ...

Fixes #792.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/801

-- Commit Summary --

  * Make extract throw for a disturbed/locked ReadableStream

-- File Changes --

    M fetch.bs (52)

-- Patch Links --

https://github.com/whatwg/fetch/pull/801.patch
https://github.com/whatwg/fetch/pull/801.diff

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