Codeigniter returning code 0 db errors without message

I have a query that is supposed to return an error message like this:

Error Number: 1451    
Cannot delete or update a parent row: a foreign key constraint fails (`sservice_dev_mercury`.`agent_accounts`, CONSTRAINT `agent_role_fk` FOREIGN KEY (`role`) REFERENCES `roles` (`id`))    
DELETE FROM `roles` WHERE `id` = '3'

This is when db_debug is true. When I turned it false and used

$error = $this->db->error();
return $error;

I get error 0 with an empty message. Is there something I am missing when trying to get the errors? This is the query:

public function delete($id){
     $this->db->trans_begin();
     $this->db->where('id',$id);
     $this->db->delete($this->table);
     $this->db->trans_complete();
     if($this->db->trans_status()===false){
        $this->db->trans_rollback();
        $error = $this->db->error();
        return $error;
     }else{
        $this->db->trans_commit();
        return true;
     }
}

Can someone take a look and see if something went wrong?