Error format

[erincandescent]

So I very much love how Nexus is a very simple solution to a ubiquitous problem in API design; but one objection I have is the Failure type. One of the last things the world needs is Yet Another JSON+HTTP Error Format

RFC 9457 (that replaces 7807) defines a common JSON error format for HTTP APIs that is growing in popularity. It's very simple and minimalist, and (IMO) does precisely what a common machine-readable error format must.

I feel that using this instead of the Failure type would align with growing practice for HTTP (and particularly JSON-based) APIs.

I don't think this would be a big change to the spec:

• The definition of Failure could be dropped,
• Instead of "Content-Type: application/json; Body: A serialized failure" the spec would say "Content-Type: application/problem+json; Body: A serialized RFC 9457 Problem report".
• Perhaps instead of the "Nexus-Operation-State` response header a specific problem type could be defined for an operation being canceled. (I think you still need it for callbacks, but there perhaps the only options are success or failure; cancelation is really just one specific failure cause anyway)

I realise this might be the kind of change thats annoying to make at this point, but I think the alternative is that people will be cursing having yet another error format to deal with for years to come.