Context is whatever makes sense to provide to a consumer to help them debug it or respond to it - the same basic idea as in the rfc under details. IMO, it can’t easily be generalized. Some APIs may have context to provide, others may not. These could be validation errors in a structured format, or backoff timings in the case of a 429.
Success is something that you can sniff for after deserializing, as IIRC Fetch API will not throw except for a network errors, even in the event of a 4XX or 5XX.
Consider something like:if(!obj.error_code){} vs if(obj.success){ }. Certainly, you could consolidate the error_code and success member, but with the sloppy truthiness of testing in Javascript, including something like that as a standard part of all responses may make sense.