Why mysqli_sql_exception is not caught by PHP?

Question

I am trying to create a php script to add new users. The username is unique, so if a new users tries to use a name already used the SQL fails. I am trying to catch the error using the following code:

<?php
include 'dbConn.php';
if (!isset($_SESSION['username'])) session_start();

$username = 'foo';
$userpass = 'foopass';
$userrole = 1;
global $mysqli;
try{
    connect();
    $mysqli -> autocommit(FALSE);
    $sqluser= "INSERT INTO users (login,password,role_id) VALUES(?,?,?); ";
    $stmt = $mysqli->prepare($sqluser);
    $stmt->bind_param("ssi", $username, $userpass , $userrole);
    $res = $stmt->execute();  //Fatal error: Uncaught mysqli_sql_exception: Duplicate entry 
    if($res){
        $mysqli->commit();
        $mysqli->autocommit(TRUE);
        echo "<p>New Account created!</p>";
    }else{
        $mysqli->rollback();
        $mysqli -> autocommit(TRUE);
        echo "<p>New Account creation Failed - Maybe the username already exists!";
    }
}catch(err){
    $mysqli->rollback();
    $mysqli -> autocommit(TRUE);
        echo "<p>New Account creation Failed - Maybe the username already exists!";
}

?>

However the code breaks in line $res = $stmt->execute(); showing a Fatal error message. How should I catch the SQL error from PHP?