application_controller.rb

require 'exception_handler'

class ApplicationController < ActionController::Base
  include YourApp::ExceptionHandler
  
end

exception_handler.rb

# this would live in Rails.root/lib/ or Rails.root/app/mixins/
module YourApp
  module ExceptionHandler
    
      def rescue_action_in_public(exception)

        case status_code(exception).to_s
        when /404/
          render '/errors/404', :status => 404
        else
          render '/errors/500', :status => 500
        end 
       
      end

  end
end