Error handling in Ajax call (jQuery) not working – Jquery ajax

I use jQuery and Ajax to pass data to a PHP / MySQLi page.
The code and the query work as intended but I can’t find a way to handle the errors.
E. g. I want to inform the user about a potential duplicate record when the data that I pass to the PHP page already exists in my database (see comment in the code below).

I tried different approaches but my error handling is always ignored or not applied.
Can someone tell me how to do this right ?

jQuery (shortened):

$('#btnSave').on('click', function(e) {
    e.preventDefault();

    var vid = $.trim($('#vid').val());
    var vid2 = $.trim($('#vid2').val());
    var vid3 = $.trim($('#vid3').val());
    var mode = 'update';
        if(vid == 'new') {
            mode = 'insert';
        }

    $.ajax({
        type: 'POST',
        url: 'updateVid.php',
        data: {
            mode: mode,
            vstId: vstId, 
            vstId2: vstId2,
            vstId3: vstId3
        },
        success: function(result){
            alert('success');
        },
        error: function() {
            alert('error'); // I am unable to retrieve this in jQuery / Ajax resp. to do anything here
        }
    });
});

PHP / MySQLi (shortened):

<?php   
    require_once 'me/config.php';

    $postData = $_POST; 
    $mode = $_POST['mode'];
    $vid = $_POST['vid'];
    $vid2 = $_POST['vid2'];
    $vid3 = $_POST['vid3'];

    $conn = new mysqli($host, $username, $password, $database);
    if($conn->connect_error) {
        die("Connection Error: " . $conn->connect_error);
    }   
    if($mode == 'update') {
        $stmt = $conn->prepare("UPDATE vids v SET v.vid2 = ?, v.vid3 = ? WHERE v.vid = ?");
        $stmt->bind_param("sss", $vid, $vid2, $vid3);
        $stmt->execute();
    } else {
        $vid = '99999999';
        $vid2 = '99XXX999';

        $stmt = $conn->prepare("SELECT vid2 FROM vids WHERE vid = ?");
        $stmt->bind_param("s", $vid);
        $stmt->execute();
        $stmt->store_result();

        if($stmt->num_rows > 0) {
            echo 'Error'; // I am unable to retrieve this in jQuery / Ajax
        } else {
            $stmt->close();

            $stmt = $conn->prepare("INSERT INTO vids (vsid, vid2, vid3) VALUES (?, ?, ?)");
            $stmt->bind_param("sss", $vid, $vid2, $vid3);  
            $stmt->execute();

            echo 'Success';
        }
    }

    $stmt->close();
    $conn->close(); 
?>

Many thanks in advance,
Tom