code: func (s *State) LookupStepErr(name string) (*Step, error) {
  	if as := s.LookupStep(name); as != nil {
  		return as, nil
  	}
  	return nil, fmt.Errorf("no step named %q", name)
  }
docs: // LookupStepErr returns the step with the supplied name, or an error if no
  // such step exists.
  //
  // If multiple steps share a name, this will return the latest registered step
  // with that name.