commento.js: show error when comment link is 404

Signed-off-by: Adhityaa Chandrasekar <adtac@adtac.in>

frontend/js/commento.js

@@ -2107,6 +2107,7 @@
       if (window.location.hash.startsWith("#commento-")) {
         var el = $(ID_CARD + window.location.hash.split("-")[1]);
         if (el === null) {
+          errorShow("The comment you're looking for no longer exists. Perhaps it was deleted by the commenter or a moderator?");
           return;
         }