Better handle ResponseEmitter closure in Emit

[Stebalien]

We need to make it clear whether the emitter is simply closed (and the sender should go away) or something actually went wrong.

Really, we should probably just log what went wrong and return a boolean where false means "closed". Currently, we return an error but it's unclear how this error should be handled. Should the caller follow up with a SetError call? (the answer is no but the API is really unclear).

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Also, we need to make sure that all emitters handle context cancellation correctly.