Better error when trying to add to nonexistent pool

fixes #2546

app/controllers/pool_elements_controller.rb

@@ -5,13 +5,13 @@ class PoolElementsController < ApplicationController
   def create
     @pool = Pool.find_by_name(params[:pool_name]) || Pool.find_by_id(params[:pool_id])
 
-    if @pool.present?
+    if @pool.present? && !@pool.is_deleted?
       @post = Post.find(params[:post_id])
       @pool.add!(@post)
       append_pool_to_session(@pool)
+    else
+      @error = "That pool does not exist"
     end
-
-    respond_with(@pool, :location => post_path(@post))
   end
 
   def destroy

app/views/pool_elements/_new.html.erb

@@ -1,4 +1,4 @@
-<%= form_tag(pool_element_path, :class => "simple_form") do %>
+<%= form_tag(pool_element_path, :class => "simple_form", :remote => true, :format => :js) do %>
   <%= hidden_field_tag "post_id", @post.id %>
 
   <div class="input">

app/views/pool_elements/create.js.erb

@@ -0,0 +1,5 @@
+<% if @error %>
+  Danbooru.error("<%= j @error.to_s %>");
+<% else %>
+  location.reload();
+<% end %>