How to catch database operation errors

I extended the model as followed for testing purpose:

PHP Code:

        $this->db->where('id', $userID);
        $error = $this->db->error();
        echo json_encode($error); die;

        $this->db->delete('users'); 


But it always returns no error. Even if I put error() after db-delete.
The response in Developer Tools is:

Code:

{"code":0,"message":""}