I believe your query is to change -1 to Exist Tag not found?
First of all, It shows -1 if not found? Well, it shouldn't. It should give a response 0. For logged in users, the ajax response is handled by WordPress core and you can't change it. But for logged out users (guests), WPUF handles that and you can change the response from wpuf-functions.php and it's wpufe_ajax_tag_search() function.