api to return error status

config.ru

$: << '.'
require 'test_app'

use Rack::ContentType
use Rack::ContentLength
run TestApp

test_app.rb

require 'sinatra/base'
require 'json'

module ErrorResponse
  def render_json(status, message_id, opt={})
    [ status, opt.merge({ 'Content-Type' => 'application/json' }), [ { 'message_id' => message_id }.to_json ] ]
  end

  def self.included(base)
    base.module_eval do
      # WWW-Authenticate response-header field MUST be included in 401
      # but, I don't know its syntax
      get('/errors/401') { render_json(401, 'client error', 'WWW-Authenticate' => 'some chalenge') }

      # Allow
      get('/errors/405') { render_json(405, 'client error', 'Allow' => 'POST' ) }

      # Proxy-Authenticate
      get('/errors/407') { render_json(407, 'client error', 'Proxy-Authenticate' => 'some challeng') }

      # TODO: 413 Retry-After (when temporary)

      (400..426).each do |status|
        get("/errors/#{status}") { render_json(status, 'client error') }
      end

      (500..510).each do |status|
        get("/errors/#{status}") { render_json(status, 'server error') }
      end
    end
  end
end

class TestApp < Sinatra::Base
  include ::ErrorResponse
end