DetachArrayBuffer can throw

[domenic]

Discovered and discussed in whatwg/html#4601.

This impacts https://streams.spec.whatwg.org/#transfer-array-buffer, and all of its transitive callers. We need to find the appropriate point at which we'd catch the errors and transform them into rejected promises or errored streams.

This only impacts byte streams, which nobody implements yet, so the urgency is not super-high here, but we should still fix it.