[streams] Consistency of return value of ReadableStreamController method call on closed/errored streams (#388)

It's minor thing, but there's some inconsistency in the reference impl.

Currently:
- close() on a close()-ed stream: throw a TypeError saying it's already closed
- close() on an errored-before-close() stream: throw a TypeError saying it's errored
- enqueue() on an errored stream: throw a TypeError saying it's errored
- enqueue() on a close()-ed but not errored stream: throw a TypeError saying it's already closed


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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/388