[JavaScript + Promises] Unhandled Promise Rejection?

Description

It appears that ApiClient#callApi will return a rejected Promise if an error is present, but API operation methods only have resolution handlers. Furthermore, I'm of the opinion that operation methods should simply return the Promise regardless of its resolved or rejected status so that consumers of the SDK can decide what to do with it.

Swagger-codegen version

2.3.1

Swagger declaration file content or url

http://petstore.swagger.io/v2/swagger.json

Command line used for generation

swagger-codegen generate \
  -i http://petstore.swagger.io/v2/swagger.json \
  -l javascript \
  -o . \
  --additional-properties usePromises=true,useES6=true

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Current:

<operationId>(<vendorExtensions.x-codegen-argList>) {
  return this.<operationId>WithHttpInfo(<vendorExtensions.x-codegen-argList>)
    .then(function(response_and_data) {
      return response_and_data.data;
    });
}

Proposed:

<operationId>(<vendorExtensions.x-codegen-argList>) {
  return this.<operationId>WithHttpInfo(<vendorExtensions.x-codegen-argList>);
}

// or...

<operationId>(<vendorExtensions.x-codegen-argList>) {
  return this.<operationId>WithHttpInfo(<vendorExtensions.x-codegen-argList>)
    .then(function(response_and_data) {
      return response_and_data.data;
    }, function(response_and_error) {
      return response_and_error;
    });
}

Start with modules/swagger-codegen/src/main/resources/Javascript/es6/ApiClient.mustache and api.mustache, then generate the petstore client using the command shown in the issue. Compare the generated operation methods with ApiClient#callApi and verify that rejected promises remain available to SDK consumers rather than becoming unhandled rejections.