[Go] generated API code swallows decoding errors

Description

If the API returns a 200, but the JSON payload is malformed, then the code Go code generated hides the decoding errors. It's more useful to surface these errors to the developer to handle.

Generated template currently with 2.4.15:

if localVarHttpResponse.StatusCode < 300 {
	// If we succeed, return the data, otherwise pass on to decode error.
	err = a.client.decode(&localVarReturnValue, localVarBody, localVarHttpResponse.Header.Get("Content-Type"));
	if err == nil { 
		return localVarReturnValue, localVarHttpResponse, err
	}
}

Proposed change:

if localVarHttpResponse.StatusCode < 300 {
	// If we succeed, return the data, otherwise pass on to decode error.
	err = a.client.decode(&localVarReturnValue, localVarBody, localVarHttpResponse.Header.Get("Content-Type"));
	return localVarReturnValue, localVarHttpResponse, err
}

Swagger-codegen version

2.4.15.
Technically, it's not a regression since 2.3.x returned generic interface{}s. That version required you to do your own decoding.

Swagger declaration file content or url

not needed, it's with all Go generated code

Command line used for generation

swagger-cli generate

Steps to reproduce

You can generate any code.

Related issues/PRs

I opened https://github.com/swagger-api/swagger-codegen/pull/10429

Suggest a fix/enhancement

https://github.com/swagger-api/swagger-codegen/pull/10429

Start with the Go code-generation template that emits the shown decode block and compare the proposed change in PR 10429. Confirm how malformed JSON from a successful response is handled, then ensure the generated client returns that decoding error; the issue does not name a file or test.