Generate static error pages from normal Rails templates.

errors_controller.rb

class ErrorsController < ApplicationController
  layout 'errors'
  caches_page :show

  def show
    render action: params[:id]
  end
end

maintenance.rake

namespace :maintenance do
  desc 'Create static versions of our custom HTTP error pages'
  task :create_error_pages => :environment do
    if Rails.application.config.action_controller.perform_caching
      session = ActionDispatch::Integration::Session.new(Rails.application)

      %w{401 404 422 500 maintenance}.each do |error|
        puts "Creating /errors/#{error}.html"
        session.get("/errors/#{error}")
      end
    else
      puts %{
        ** Error: config.action_controller.perform_caching is false, which means these pages wouldn't be created. Try this instead:

                  RAILS_ENV=production #{File.basename $0} #{ARGV.join(' ')}
      }.strip_heredoc.strip
    end
  end
end

routes.rb

# ...
resources :errors, only: :show
# ...