ext JS Ext.form.FormPanel 如何显示提交后服务器返回的错误信息？

var f = new Ext.form.FormPanel({
		renderTo:"gatherDiv",
		title:"成果搜索信息录入框",
		width:400,
		autoHeight:true,
		labelWidth:60,
		labelAlign:"right",
		frame:true,
		defaults:{xtype:"textfield",width:180},
		items:[{name:"username",fieldLabel:"姓名"},
				{name:"password",fieldLabel:"密码",inputType:"password"},
				{name:"email",fieldLabel:"电子邮件"},
				{xtype:"textarea",name:"intro",fieldLabel:"简介"}
		],
		buttons:[{text:"提交",handler:function(){
			var addUrl= $("#addUrl").val();
			alert(addUrl);
			f.form.submit({
				waitTitle:"请稍候",
				waitMsg:"正在提交表单数据，请稍后......",
				url:addUrl,
				method:"POST",
				success:function(form,action){
					alert("添加成功！"+ action.result.errMsg);
				},
				failure:function(form,action){
					//+ action.result.errMsg
					alert("添加失败！"+ action.result.errMsg);
				}
			});
		}},
		{text:"重置",handler:function(){
			f.form.reset();
		}}]
	});

页面显示图如下：

jsp页面内容：

<div id="gatherDiv"></div>
<input type="hidden" id="addUrl" value="<c:url value="/fruitAdd.do"/>">

java后台代码：

@RequestMapping("/fruitAdd")
	public void fruitAdd(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
		result.put("success", false);
		result.put("errMsg", "exist");

		SCPUtil.outputJSON(result, response);
	}

SCPUtil.outputJSON(result, response);代码如下：

	public static void outputJSON(JSONObject json, HttpServletResponse response)
	throws IOException, ServletException
	{
		response.setContentType("text/html;charset=utf-8");
		PrintWriter writer = response.getWriter();
		json.write(writer);
	}

点击“提交”结果如下：