Show error message with register_setting()

does anyone know how to incorporate an error message using register_setting()? I have a callback function that checks to see if a value of an option already exists and if so, prevents the user from overriding it. I can prevent the override, but I can’t display an error message. Additionally, WP shows the “settings saved” message.

This is the code:

// Sanitize and validate input
function azs3_upload_pages_validate($input) {

    $upload_pages = '';
    $upload_pages .= get_option('azs3_upload_pages');

    if ( isset($_POST['delete_upload_page']) && '' != $_POST['delete_upload_page'] ) {
        $upload_page = $_POST['delete_upload_page'];
        if( '' != $upload_pages ) {
            $upload_pages = explode (',', $upload_pages);
            foreach ($upload_pages as $k => $v) {
                if ($v == $upload_page)
                unset($upload_pages[$k]);
            }
            $upload_pages = implode(',',$upload_pages);
        }
    } else {
        if( '' != $upload_pages ) {
            $assigned_pages = explode (',', $upload_pages);
            if( in_array($input, $assigned_pages) ) {
                $errors = new WP_Error();
                $errors->add('pageexists', __('Page already exists.'));
                return $upload_pages;
            } else {
                $upload_pages .= ','.$input;
            }
        } else {
            $upload_pages = $input;
        }
    }
    return $upload_pages;
}