How to show error in a div if an error occurred in jquery?

How to show error in a div if an error occurred in jquery?

My PHP code is this;
if ($mybb->input['action'] == 'addcomment')
{
if (!empty($mybb->user['kicked_by']))
{
echo 'You do not have permissions to chat.';
exit;
}
$query = $db->query("
SELECT message FROM ".TABLE_PREFIX."chatbox
WHERE `message` = '{$mybb->input['message']}' AND
uid='{$mybb->user['uid']}'
");
$check = $db->fetch_field($query, "message");
if ($check)
{
echo 'The same message has been posted previously.';
exit;
}
if ($mybb->input['private'] == "1")
{
$private = '1';
}
else
{
$private = '0';
}
$cuid = intval($mybb->user['uid']);
$message = $db->escape_string($mybb->input['message']);
$insert_array = array(
"uid" => $cuid,
"message" => $message,
"edited" => '',
"approved" => '1',
"private" => intval($private),
"dateline" => TIME_NOW,
"ip" => get_ip()
);
$db->insert_query("chatbox", $insert_array);
}
and jquery I used for this is;
$(".textbox2").live('keypress', function (e) {
if (e.keyCode == 13 && !e.shiftKey) {
e.preventDefault();
var message=$("#message").val();
if (message == '')
{
alert("Message is missing!!");
return;
}
if ($("#private").is(':checked') ? 1 : 0)
{
var private=$("#private").val();
}
$.ajax({
type:"post",
url:"chat_insert.php",
data:"message="+message+"&private="+private+"&action=addcomment",
success:function(data){
showComment();
document.getElementById('message').value='';
document.getElementById('message').focus();
document.getElementById('private').checked='';
},
onComplete:function(data){
alert("Error: "+data);
}
});
}
});
But I've no clue as to how and what I missed there that's why no error
message is outputting in neither alert nor I try to use div id in jquery.
Please help