Add AbortError or EncodingError exception as argument for rejected flush() promises

[chcunningham]

Alligns with how promises are rejected in the ImageDecoder proposal (#152)

Saying why the promise was rejected seems helpful. Not strictly necessary, since they may know why via other means (e.g. EncodingError would also be fired via the error callback).

[chcunningham]

Triage note: marking 'breaking' as this changes what a promise is rejected with. Having said that, the break is very minor (most folks will probably ignore this value).