Jquery ajax执行顺序 返回自定义错误信息(实例讲解)

function TestAjax(){
            var UserName = $("#txtUserName").val();
            $.ajax(
            {
                url: "AjaxCheckUserName.htm",
                async: false,
                success: function (data) {
                    alert(data);
                }
            });
            alert('Test');
            $.ajax({
                url: "AjaxHandler.ashx",
                async: false,
                data: "UserName=" + UserName,
                success: function (data) {
                    $("#divAjax").html(data);
                },
                error: function (msg) {
                    alert(msg.responseText);
                }
            });
        }

error: function (msg) {
                    alert(msg.responseText);
                }

 context.Response.StatusCode = 500;
 context.Response.Write("请输入用户名");

string sUserName = context.Request.QueryString["UserName"];
            if (!string.IsNullOrEmpty(sUserName))
            {
                context.Response.Write(string.Format("Hello {0}!", sUserName));
            }
            else
            {
                context.Response.StatusCode = 500;
                context.Response.Write("请输入用户名");
            }