favorites: fix error handling.

* Return HTTP 422 instead of HTTP 500 on "You have already favorited
  this post" errors.

* Log unexpected errors in Ajax requests to the console.

app/controllers/application_controller.rb

@@ -26,6 +26,12 @@ class ApplicationController < ActionController::Base
 
   protected
 
+  def self.rescue_with(*klasses, status: 500)
+    rescue_from *klasses do |exception|
+      render_error_page(status, exception)
+    end
+  end
+
   def show_moderation_notice?
     CurrentUser.can_approve_posts? && (cookies[:moderated].blank? || Time.at(cookies[:moderated].to_i) < 20.hours.ago)
   end

app/controllers/favorites_controller.rb

@@ -2,6 +2,7 @@ class FavoritesController < ApplicationController
   before_action :member_only, except: [:index]
   respond_to :html, :xml, :json, :js
   skip_before_action :api_check
+  rescue_with Favorite::Error, status: 422
 
   def index
     if params[:tags]

app/views/static/error.js.erb

@@ -1,2 +1,10 @@
+var klass = <%= raw @exception.class.to_s.to_json %>;
 var message = <%= raw @message.to_json %>;
-Danbooru.Utility.error(message);
+var backtrace = <%= raw @backtrace.to_json %>;
+
+<% if @expected %>
+  Danbooru.Utility.error(message);
+<% else %>
+  console.error(klass, message, backtrace);
+  Danbooru.Utility.error(message);
+<% end %>