MUST use problem json for errors

The content type for 4xx and 5xx status codes MUST be application/problem+json.

Invalid Example

The content type for the 503 response in this example incorrectly uses the application/json content type.

responses:
   '503':
     description: ...
     content:
       application/json:
         schema:
           $ref: ../models/Problem.yaml

The content type for the 503 response in this example correctly uses the application/problem+json content type.

responses:
   '503':
     description: ...
     content:
       application/problem+json:
         schema:
           $ref: ../models/Problem.yaml