There's nothing in the protocol, but in general, you should not assume non-2xx status codes have parseable payloads (more specifically, you should not assume the format of a non-200 status code).

Reason being that any intermediary step in the chain of resolving your request can return the response. If your load balancer has fallen over? You'll get a 500 with a plaintext body. Don't parse that as JSON.

(Technically, any intermediary step might also return you a 2xx with a non-parseable body, but that scenario is far, far more rare... Mostly see it in completely misconfigured servers where instead of getting the service you're trying to talk to you get the general "Set up your site" landing page).