return Response()->json(array('error' => '404 car type not found'), 404);

$('document').ready(function () {
$('#car_type').bind('changed.bs.select', function () {
    $.ajax({
        type: 'POST',
        url:'carclass/'+$('#car_type').val(),
        dataType: 'json',
        error: function(XMLHttpRequest, textStatus, errorThrown){
            alert('status:' + XMLHttpRequest.status + ', status text: ' + XMLHttpRequest.statusText);
        },
        success: function( json ) {
           $.each(json, function(i, obj){
                $('#car_class').append($('<option>').text(obj.name).attr('value', obj.id));
           });
           $('#car_class').selectpicker('refresh');
        }
});
 });
 });

GET http://localhost:8000/ads/cartype/2 404 (Not Found)

error:function (xhr, ajaxOptions, thrownError){
    if(xhr.status==404) {
        alert(thrownError);
    }
}

error:function (xhr, ajaxOptions, thrownError){
    if(xhr.status==404) {
        alert(thrownError);
    }
}