gistfile1.rb

# app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  rescue_from(ActionController::RoutingError) {
    render :template => 'errors/404'
  }
end

# config/initializers/show_exceptions.rb
require 'action_dispatch/middleware/show_exceptions'

module ActionDispatch
  class ShowExceptions
    private
      def render_exception_with_template(env, exception)
        body = ErrorsController.action(rescue_responses[exception.class.name]).call(env)
        log_error(exception)
        body
      rescue
        render_exception_without_template(env, exception)
      end
      
      alias_method_chain :render_exception, :template
  end
end

# app/controllers/errors_controller.rb
class ErrorsController < ApplicationController
  ERRORS = [ 
    :internal_server_error,
    :not_found,
    :unprocessable_entity
  ].freeze
  
  ERRORS.each do |e|
    define_method e do
      respond_to do |format|
        format.html { render e, :status => e }
        format.any { head e }
      end
    end
  end
end