How to show error messages from models?

[eluser]Alhazred[/eluser]
I've tried but that didn't help.
I've solved in this way

Renamed the model file from authentication.php to authentication_model.php
Model's code

Code:

class Authentication_model extends CI_Model
{
function __construct()
{
  parent::__construct();
}

...
}

Controller's code

Code:

$this->load->model('authentication/Authentication_model');
$this->Authentication_model->init($username,$password);
...

In other words I have added _model to the model class name and filename.
Is it required to use _model?
From the user guide seems it is not, but without the code doesn't work.