Unbranded CodeGen should generate error models instead of reusing the model from core library

[lirenhe]

For JS modular under unbranded scenario, we need to generate the error model from the following definition:

@error
model ApiError {
  /** A machine readable error code */
  code: string;

  /** A human readable message */
  // https://github.com/microsoft/OpenAPI/blob/main/extensions/x-ms-primary-error-message.md
  @OpenAPI.extension("x-ms-primary-error-message", true)
  message: string;
}

Please also verify we could map the error by status code correctly.

model Standard4XXResponse extends ApiError {
  @minValue(400)
  @maxValue(499)
  @statusCode
  statusCode: int32;
}

[MaryGao]

I will scope this issue to p1 part mentioned here so updated the priority here, for p0 part we would cover in this existing issue: #2917.

@timovv will help drive the p1 part from core to codegen side.

[lirenhe]

@MaryGao, for unbranded, please teste p0 scenario with the generated service to ensure it works E2E.