Doing a validation check on a AJAX post and returning the error message

I have an AJAX post that does this

  $.ajax({
                    type: "POST",
                    url: "@MyWebSite.Url/myController/myView",
                    contentType: "application/json; charset=utf-8",
                    data:
                    JSON.stringify({ myModel: myData }),
                    dataType: "json",
                    traditional: true,
                    success: function () {
                        alert('Success!');
                    },
                    error: function () {
                        alert('Error! ');
                    }

My controller does the validation check but it is not correctly returning the error message.
This is what my controller looks like

 if (totalQty < part.QtyInItem)
                    {
                        //ModelState.AddModelError("", "There is " + part.QtyInItem + " of Part " + part.PartName + " used in item " + part.ItemID + " but you only handled " + totalQty + ". Please verify you are handling all parts used in the item.");
                        //RedirectToAction("myControler", myModel);
                        return this.Json(new { success = false, message = "There is " + part.QtyInItem + " of Part " + part.PartName + " used in item " + part.ItemID + " but you only handled " + totalQty + ". Please verify you are handling all parts used in the item." });
                    }

When I tried adding an error to the model state it just returned “ERROR!” and not the error message I had associated with it. And when I try doing the this.JSON return it returns “success” to the view and not the error message.

How can I do this validation check for my AJAX post