Check if feed is fetched successfully

fetch_feed() returns a WP_Error() if the feed in question does not exist or fails for whatever reason. If we’re only checking for failure we can check if the returned object is_wp_error().

if ( ! is_wp_error( $rss ) ){
    // Assume the best, show feed
}
else{
    // Assume the worst, show message
}

If the feed does return valid but is empty, we can do what you’re currently doing which is to try and get items from the valid feed:

$maxitems = $rss->get_item_quantity(1);

If $maxitems is empty or 0, then we know the feed is a valid, but empty.