How to use the ShoppingCartValidateProductUpdate hook

[yomyyo]

I am trying to validate custom fields for my item, but whether or not the custom field is valid, I will be thrown an error. If it's an invalid field it'll say  "Please correct the following errors before continuing: This email address is already in use." However if the user gives a valid field, it will just give "Please correct the following errors before continuing:" without anything after.

I have the hook set up as so

function checkUser($vars){
    $postData = array(
        'data' => array(
            'email' => $vars['customfield'][6],
        )
    );
    
    $ch = curl_init();

    if($ch == false){
        throw new Exception('failed to init');
    }

    curl_setopt_array($ch, array(
        CURLOPT_URL => myUrlGoesHere,
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_POST => true,
        CURLOPT_HEADER => 0,
        CURLOPT_POSTFIELDS => http_build_query($postData),
    ));
    
    //Save the result from the request
    $output = curl_exec($ch);
    $res = json_decode($output);

    curl_close($ch);
    console_log($res);
    if($res){
        $message = "This email address is already in use.";
        return $message;
    }
}

Is there a value I need to return using this hook.

Thank you!