Handle validation rule message inside controller

Hi there, i've start to learn cake php yesterday, and i have a question. I've declared validation array inside my model class like:

    

public $validate = array(

    'username' => array(

        'alphaNumeric' => array(

            'rule' => 'alphaNumeric',

            'required' => true,

            'message' => 'username required'

         ),

         'unique' => array(

            'rule' => 'isUnique',

            'required' => 'create',

            'message' => 'Username already used'

         )

        ));

When i add a new record to my database by using add(), everithing works fine, and alert message are showed automatically. How can i intercept this error message firing inside add() controller function? because if a message is fired, i would perform an action (and not simply show message)

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