How to gracefully handle MySQL errors in a JSON request?

New to AJax/JSON and need assistance in handling MySQL errors on the called PHP page.  

All of the examples I've found online do a "die" if the query fails or no records are returned.  I want to gracefully handle errors by showing an alert if the call fails. (This is also a huge advantage to me in debugging.)  The text I want to show is generated on the PHP page.

Rough code follows.

Thanks!
Bruce


javascript function code:

	var xmlhttp;
	xmlhttp = new XMLHttpRequest();
	
	xmlhttp.onreadystatechange=function() {
		if (xmlhttp.readyState==4 && xmlhttp.status==200) {
		    txt = xmlhttp.responseText;
         		var R = JSON.parse(txt);
// load variables, display info, etc.
	} 
	
	xmlhttp.open("GET","ajax_user_data.php?id=" + rqstr, true);
	xmlhttp.send();

PHP page code:
<?php
header("content-type:application/json");
session_start();
include_once("adiInclude.php");

$id = $_GET['id'];
$txt = '';
$connAJAX = ConnectDB();   // function is in include file

$sql = 'SELECT * FROM users WHERE user_id = ' . $id;

$qry = mysql_query($sql, $connAJAX);

if (!$qry) {
	$txt = 'JSON Users query failed: ' . mysql_error($connAJAX) . " SQL: " . $sql;
		
} elseif (mysql_numrows($qry) == 0) {
	$txt = "JSON Invalid User ID: " . $id;
	mysql_free_result($qry);
	
} else {
	$row = mysql_fetch_assoc($qry);
	foreach($row as $i => $v) {
		if ($i == "user_id" || $i == "user_cam" || $i == "user_country" || $ == "user_via") {
			$txt .= ', "' . $i . '":' . $v . '';
		} else {
			$txt .= ', "' . $i . '":"' . $v . '"';
		}
	}
	$txt = '{ ' . substr($txt, 2) . ' }';
	
	mysql_free_result($qry);	
}
mysql_close($connAJAX);
echo $txt;	
exit();
?>

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