Ajax 表单提交成功，但出现警报错误

我创建了一个提交到 php 页面的 AJAX 表单。此表格已成功提交，我在我的数据库中获得了电子邮件地址，并且我有一封发送的确认电子邮件。

但是，我收到一条警报消息，说"错误|"，所以很明显它来自这个：

error: function (xhr, textStatus, errorThrown) {
            alert(textStatus + "|" + errorThrown);

我不确定如果它有效，为什么会抛出错误。另一件事，此表单正在重新加载页面。我已经event.preventDefault();了，那么为什么要重新加载页面？

我感谢任何帮助。

<form action="" method="POST">
  <input type="email" id="footer-grid1-newsletter-input" placeholder="Your Email Address">
  <input type="submit" id="footer-grid1-newsletter-submit" name="submit">
</form>

$(document).ready(function(){ 
    $("#footer-grid1-newsletter-submit").on("click", function () {
        event.preventDefault();
        var newsletter_email = $("#footer-grid1-newsletter-input").val();
        $.ajax({ 
            url: "newsletterSend.php", 
            type: "POST",
            data: {
                "newsletter_email": newsletter_email
            },
            success: function (data) {
            //  console.log(data); // data object will return the response when status code is 200
                if (data == "Error!") {
                    alert("Unable to insert email!");
                    alert(data);
                } else {
                    /*$(".announcement_success").fadeIn();
                    $(".announcement_success").show();
                    $('.announcement_success').html('Announcement Successfully Added!');
                    $('.announcement_success').delay(5000).fadeOut(400);*/
                }
            },
            error: function (xhr, textStatus, errorThrown) {
                alert(textStatus + "|" + errorThrown);
                //console.log("error"); //otherwise error if status code is other than 200.
            }
        });
    });
});
ini_set('display_errors', 1);
error_reporting(E_ALL);
$newsletter_email = $_POST['newsletter_email'];
try {
    $con = mysqli_connect("localhost", "", "", "");
    if (mysqli_connect_errno()) {
        printf("Connect failed: %s'n", mysqli_connect_error());
        exit();
    }
    $stmt = $con->prepare("INSERT INTO newsletter (email, subscribed) VALUES (?, NOW())");
        if ( false===$stmt ) {
            die('Newsletter email prepare() failed: ' . htmlspecialchars($con->error));
        }
    $stmt->bind_param('s', $newsletter_email);
        if ( false===$stmt ) {
            die('Newsletter email bind_param() failed: ' . htmlspecialchars($stmt->error));
        }
    $stmt->execute();
        if ( false===$stmt ) {
            die('Newsletter email execute() failed: ' . htmlspecialchars($stmt->error));
        }
 } catch(Exception $e) {
    die($e->getMessage());
}