When the client cannot unmarshal an error response, the response is lost

[taylor-sutton]

Example generated code: https://github.com/Clever/wag/blob/e1e9b5bbdd8c9b945e00c2be9d4fe5d19ebda906/samples/gen-go/client/client.go#L327-L330

This is is perhaps a bit tricky to handle better if we want to continue with allowing the response body to be streamed?