application_controller.rb

class ApplicationController < ActionController::Base
  # ...

  unless Rails.application.config.consider_all_requests_local
    rescue_from Exception, with: lambda { |exception| render_error 500, exception }
    rescue_from ActionController::RoutingError, ActionController::UnknownController, ::AbstractController::ActionNotFound, ActiveRecord::RecordNotFound, with: lambda { |exception| render_error 404, exception }
  end

  private
  def render_error(status, exception)
    respond_to do |format|
      format.html { render template: "errors/error_#{status}", layout: 'layouts/application', status: status }
      format.all { render nothing: true, status: status }
    end
  end

  # ...
end

error_404.html.haml

%h2 404
%div
  %h3 We're sorry
  %p
    The content that you requested could not be found.
  %p
    You tried to access '#{@not_found_path}', which is not a valid page.
  %p
    Want to
    %a{href: root_path} go back to our home page
    and try again?

errors_controller.rb

class ErrorsController < ApplicationController
  def error_404
    @not_found_path = params[:not_found]
  end

  def error_500
  end
end

generate.sh

rails generate controller errors error_404 error_500

remove_routes.rb

  get "errors/error_404"
  get "errors/error_500"

routes.rb

  unless Rails.application.config.consider_all_requests_local
    match '*not_found', to: 'errors#error_404'
  end