A bunch of clientError that were called as serverError

Calling serverError with a clientError status code (4xx) means it will
automatically default to Internal Server Error (500) which is in the
server error status code range (5xx). That is undesirable.

plugins/Favorite/actions/showfavorites.php

@@ -130,8 +130,8 @@ class ShowfavoritesAction extends Action
         }
 
         if($this->page > 1 && $this->notice->N == 0){
-            // TRANS: Server error when page not found (404)
-            $this->serverError(_('No such page.'),$code=404);
+            // TRANS: Client error when page not found (404)
+            $this->clientError(_('No such page.'), 404);
         }
 
         return true;