I recently made this custom error message option for my codeigniter 3.0-dev applicaiton. Hope this helps anyone out there.
https://gist.github.com/abdmaster/7287962
To use it (example),
$this->form_validation->set_rules('name','Name','required|alpha',array('required' => 'Please fill the field %s .');`
It will work with Base models like jamierumbelow's MY_Model. In your model, you do something like this:
public $validate = array(
'display_name' => array(
'field' => 'display_name',
'label' => 'Display Name',
'rules' => 'trim|required|xss_clean|valid_fullname|is_unique[users_model.display_name]',
'error_msg' => array(
'is_unique' => 'The name in %s is already being used by someone.',
),
),
);
Rest are how we are use normally. Hope these examples are enough.
I haven't tried in v2.1.x but hopefully this will work. Maybe have to do some minor adjustments.