无法从AJAX获取自定义错误消息

Question

我正在尝试通过AJAX编辑我的购物车，一旦我点击编辑，它会检查STOCK中的项目，如果库存在那里它更改了购物车并且我收到了消息，但是如果库存少了，我没有得到自定义消息，所以关键是，我只能得到成功和错误（内部服务器错误）消息，无法获取自定义消息少存货

jquery代码：

$.ajax({
             type:     "post",
             url:      "/devilmaycry/register?action=editcart",
             data:     {pid:pid,quant:quant,spid:spid,cartid:cartid},
             success:  
                   function() 
                    {
                        if (xhr.responseText=="less stock")
                            alert("Stock is less");
                        else
                         alert("Item quantity has been changed in cart");
                    },
                  error:
                     function(xhr) 
                      {
                           if(xhr.responseText=="Internal server error")
                            alert("item cannot be added , internal server error");
                      } 
              }); 
             return false; 
         });

servlet代码：

else if(n.equals("editcart"))
  {
      int pid = Integer.parseInt(req.getParameter("pid"));
      int spid = Integer.parseInt(req.getParameter("spid"));
      int cartid = Integer.parseInt(req.getParameter("cartid"));
      int quant = Integer.parseInt(req.getParameter("quant"));


      k=o.editcart(cartid,pid,spid,quant);

     if(k==2)
      {
        res.setStatus(HttpServletResponse.SC_ACCEPTED);
        pw.write("less stock");
      } 
     else if(k==0)
      {
        res.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
        pw.write("Internal server error");          
      }
  }

i haven't introduced the database logic because i have checked and it is working fine