setting form validation error messagesin config file

Hi, 
I am using CI 2. I am trying to set form validation setting in config/form_validation.php. The Problem is I can't get my error messages to show up. It returns default messages and for my callback is_unique_email function it is returning "Unable to access an error message corresponding to your field name."

Here is config/form_validation.php

PHP Code:

$config = array(
'test' => array(
 
               array(
                     'field'   => 'email', 
                     'label'   => 'Email Address', 
                     'rules'   => 'required|valid_email|is_unique_email',
                     'errors' => array(
                'required' => 'field is required',
                'valid_email' => 'email is not valid',
                'is_unique_email' => 'email exist'
                  )
  
            )); 


My application/MY_Form_validation.php

PHP Code:

class MY_Form_validation extends CI_Form_validation{    
     function __construct($config = array()){
          parent::__construct($config);
     }
     public function is_unique_email($email){

         return false;
     }
} 


and my Controller:

PHP Code:

   public function form_test()
    {

        if ($this->input->post() && $this->form_validation->run('test')) { 
           
            echo true;
        }
        else {
            echo validation_errors();
            echo form_open("temp/form_test");
            echo form_input('email');
            echo form_submit('submit');
            echo form_close();
        }
    } 


 
Thanks in advance