Fix #5146: Adding an existing favorite to favorite groups leads to an error.

Show "Favgroup already contains post XXX" error when trying to add a
post to a favgroup that already contains that post.

app/controllers/favorite_groups_controller.rb

@@ -55,6 +55,8 @@ class FavoriteGroupsController < ApplicationController
   def add_post
     @favorite_group = authorize FavoriteGroup.find(params[:id])
     @post = Post.find(params[:post_id])
-    @favorite_group.add!(@post)
+    @favorite_group.add(@post)
+
+    respond_with(@favorite_group)
   end
 end