[Rails 4.x] エラーページの作り方 ref: http://qiita.com/tsumekoara/items/88815b94f63608a801a8

file0.txt

module ErrorHandlers
  extend ActiveSupport::Concern

  included do
    rescue_from Exception, with: :rescue500
    rescue_from ApplicationController::Forbidden, with: :rescue403
    rescue_from ApplicationController::IpAddressRejected, with: :rescue403
    rescue_from ActionController::RoutingError, with: :rescue404
    rescue_from ActiveRecord::RecordNotFound, with: :rescue404
  end

  private
  def rescue403(e)
    @exception = e
    render 'errors/forbidden', status: 403
  end

  def rescue404(e)
    @exception = e
    render 'errors/not_found', status: 404
  end

  def rescue500(e)
    @exception = e
    render '/errors/internal_server_error', status: 500
  end
end

file1.txt

class Forbidden < ActionController::ActionControllerError; end
class IpAddressRejected < ActionController::ActionControllerError; end

include ErrorHandlers if Rails.env.production? or Rails.env.staging?

file2.txt

get '*anything' => 'errors#routing_error'

file3.txt

class ErrorsController < ApplicationController
  def routing_error
    raise ActionController::RoutingError, "No route matches #{request.path.inspect}"
  end
end