code: func RollbackError(err error) error {
  	if err == nil {
  		return nil
  	}
  	if rerr, ok := err.(*rollbackErr); ok {
  		return rerr.rollbackErr
  	}
  	return nil
  }
docs: // RollbackError takes an error value returned by Apply and returns the error, if any,
  // that occurred when attempting to roll back from a failed update. Applications should
  // always call this function on any non-nil errors returned by Apply.
  //
  // If no rollback was needed or if the rollback was successful, RollbackError returns nil,
  // otherwise it returns the error encountered when trying to roll back.