[0.0.1] Simplify openapi client responses

[Tzrlk]

Generically, it could be done by replacing JSON200, JSON400, etc with JSON, e.g. JSONApiError.
Then there'd be fewer things that need checking.

Might end up having to customise the templates even further to fit a standard generic structure.

[Tzrlk]

Due to the use of the genResponseUnmarshal template helper when parsing responses, It'll be impossible to revise the response generation without essentially just replacing it. Might be possible with some annoying loop logic and conditionals in the template.

[Tzrlk]

Correction, not possible if any of the schemas contain advanced logic such as anyOf or oneOf relationships.