def error_state(event_name, error)
      transition = transitions.select{|t| t.is_error_transition_for?(event_name, error) }.first
      transition ? transition.to : nil
    end
def error_state(event_name, error)
      transition = transitions.select{|t| t.is_error_transition_for?(event_name, error) }.first
      transition ? transition.to : nil
    end
Returns the error state for the subject for event_name and error