protected function _validateTasks()
    {
        foreach ($this->_taskMap as $taskName => $task) {
            $class = App::className($task['class'], 'Shell/Task', 'Task');
            if (!class_exists($class)) {
                throw new RuntimeException(sprintf(
                    'Task `%s` not found. Maybe you made a typo or a plugin is missing or not loaded?',
                    $taskName
                ));
            }
        }
    }