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).
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).
Alligns with how promises are rejected in the ImageDecoder proposal (https://github.com/w3c/webcodecs/pull/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).