Custom error pages in Rails

application.rb

module MyApp
  class Application < Rails::Application
    # ...
    config.exceptions_app = self.routes

    config.action_dispatch.rescue_responses.merge!(
      "RDS::ResourceNotFound" => :not_found,
    )
    # ...
  end
end

errors_controller.rb

class ErrorsController < ApplicationController
  def show
    render status_code.to_s, status: status_code
  end

  private

  def status_code
    params[:code].to_i
  end
end

routes.rb

controller :errors do
  get ":code", to: :show, constraints: {code: /\d+/}
end