Current  (original response code and message wrapped as big string)
http-status: 500
body: ".... 401 ..... Not found "   

Wanted: 

http-status: 500

body:

{ "dmi-response":
   { "http-code": "401",
     "body": "not found"}
}

Question Issue: can we use a well defined json object as error response body with 500 response (defined in OpenApi.yaml)

already defined structure for ALL errors like this:

{
  "status": "401",
  "message": "some message",
  "details": "{internal json}"
}

Solution:

ensure org.onap.cps.ncmp.api.impl.exception.ServerNcmpException#ServerNcmpException (maybe extend?) can store dmi response status and body object as separate  properties (if need create a httpRequestException or similar)

see org.onap.cps.ncmp.api.impl.NetworkCmProxyDataServiceImpl#handleResponse 

create another method like of org.onap.cps.ncmp.rest.exceptions.NetworkCmProxyRestExceptionHandler#buildErrorResponse  -> wrapDmiErrorResponse(ServerNcmpException) to return a reponseEntity with code 500 and an error 'string' that is the proposed json above

Consider use HTTP 502:

"502 Bad GatewayThe server was acting as a gateway or proxy and received an invalid response from the upstream server.^" so we can have a dedicated format and not mix them up with internal server errors!