code: def unsubscribe
        return true if @subscription_id.nil?
  
        resp = ews.unsubscribe(@subscription_id)
        rmsg = resp.response_messages.first
        if rmsg.success?
          @subscription_id, @watermark = nil, nil
          true
        else
          raise EwsSubscriptionError, "Could not unsubscribe: #{rmsg.code}: #{rmsg.message_text}"
        end
      end
docs: Unsubscribe this folder from further Exchange events.
   @return [Boolean] Did we unsubscribe successfully?