<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def existing_choice_control(self):
 """ It controls errors. It generates an error message if zero or more than one channels are selected. """

        self.current.task_data['existing'] = False
        self.current.task_data['msg'] = _(u"You should choose just one channel to do operation.")
        keys, names = self.return_selected_form_items(self.input['form']['ChannelList'])
        if len(keys) == 1:
            self.current.task_data['existing'] = True
            self.current.task_data['target_channel_key'] = keys[0]