How to catch database operation errors

Hi

I try to catch an error if something with the database operations goes wrong.
I tried many variants of the following code but $data always returns true (it doesn't matter if the database operation produces an error or not).

What do I have to do to catch an error if database operation went wrong?

Model

PHP Code:

<?php
class Users_model extends CI_Model {
    public function deleteUser($userID)
    {
        $this->db->where('id', $userID);
        $result = $this->db->delete('users');
        return $result;
} 


Controller

PHP Code:

<?php
class Users extends CI_Controller {

    function __construct()
    {
        parent::__construct();
        $this->load->model('users_model');
    }

    public  function deleteUser()
    {
        $userID = $this->input->post('user_id');
        $data = $this->users_model->deleteUser($userID);
        echo json_encode($data);
    }
} 


View (see the "<" for var user_id. This should generate an error because the user with such an id cannot be found)

PHP Code:

            // Delete user button
            $('.btn-delete-user').on('click', function(){
                var user_id                = "<" + $(this).data("id");
                $.ajax({
                    type : "POST",
                    url  : "<?php echo site_url('users/deleteUser')?>",
                    dataType : "JSON",
                    data : {user_id:user_id},
                    success: function(data){
                        loadUsers();
                    },
                    error : function() {
                        alert("Some error happened");
                    }
                });
                return false;
            }); 


Kind regards
Alain