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

[lirenhe]

Describe the issue or request
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;
}