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

app/models/favorite_group.rb

@@ -132,15 +132,15 @@ class FavoriteGroup < ApplicationRecord
     Post.joins("JOIN (#{favgroup_posts.to_sql}) favgroup_posts ON favgroup_posts.post_id = posts.id").order("favgroup_posts.favgroup_index ASC")
   end
 
-  def add!(post)
+  def add(post)
     with_lock do
-      update!(post_ids: post_ids + [post.id])
+      update(post_ids: post_ids + [post.id])
     end
   end
 
-  def remove!(post)
+  def remove(post)
     with_lock do
-      update!(post_ids: post_ids - [post.id])
+      update(post_ids: post_ids - [post.id])
     end
   end
 

app/models/post.rb

@@ -537,12 +537,12 @@ class Post < ApplicationRecord
         in "-favgroup", name
           favgroup = FavoriteGroup.find_by_name_or_id!(name, CurrentUser.user)
           raise User::PrivilegeError unless Pundit.policy!(CurrentUser.user, favgroup).update?
-          favgroup&.remove!(self)
+          favgroup&.remove(self)
 
         in "favgroup", name
           favgroup = FavoriteGroup.find_by_name_or_id!(name, CurrentUser.user)
           raise User::PrivilegeError unless Pundit.policy!(CurrentUser.user, favgroup).update?
-          favgroup&.add!(self)
+          favgroup&.add(self)
 
         end
       end
@@ -632,7 +632,7 @@ class Post < ApplicationRecord
 
     def remove_from_fav_groups
       FavoriteGroup.for_post(id).find_each do |favgroup|
-        favgroup.remove!(self)
+        favgroup.remove(self)
       end
     end
   end

app/views/favorite_groups/add_post.js.erb

@@ -1,2 +1,7 @@
-Danbooru.notice("Added post to favorite group <%= escape_javascript(@favorite_group.pretty_name) %>");
+<% if @favorite_group.errors.any? %>
+  Danbooru.notice("<%= j @favorite_group.errors.full_messages.join("; ") %>");
+<% else %>
+  Danbooru.notice("Added post to favorite group <%= j @favorite_group.pretty_name %>");
+<% end %>
+
 $("#add-to-favgroup-dialog").dialog("close");