forum posts: raise privilege error when viewing restricted topics.

Raise a privilege error when trying to view a restricted topic instead
of handling it in the controller. This way error handling is standardized.

app/controllers/forum_posts_controller.rb

@@ -76,24 +76,7 @@ private
   end
 
   def check_min_level
-    if CurrentUser.user.level < @forum_topic.min_level
-      respond_with(@forum_topic) do |fmt|
-        fmt.html do
-          flash[:notice] = "Access denied"
-          redirect_to forum_topics_path
-        end
-
-        fmt.json do
-          render json: nil, :status => 403
-        end
-
-        fmt.xml do
-          render xml: nil, :status => 403
-        end
-      end
-
-      return false
-    end
+    raise User::PrivilegeError if CurrentUser.user.level < @forum_topic.min_level
   end
 
   def check_privilege(forum_post)