[REQ] Swift5 client: support error response schema

Is your feature request related to a problem? Please describe.

When an error occurs on the server, the server responds with an error object which is specified in the OpenAPI file. Unfortunately, the swift client does not take into account this and return a parse error instead because it did not expect that object.

The bug https://github.com/OpenAPITools/openapi-generator/issues/5504 describes this well

Describe the solution you'd like

I'd like the swift client to take into account the schema defined for error responses and to parse the response according to that schema, so the response object can be used just like for 200 responses

Describe alternatives you've considered

Right now, i end up creating code in my error handling code to parse the object manually.

Start with the Swift client generation path and the OpenAPI file's error response schemas, using issue #5504 for context. Trace how successful responses are parsed and identify where an error object instead becomes a parse error. Done means generated Swift clients parse documented error responses into usable response objects rather than failing during parsing.