render_error.rb

module RenderError
  extend ActiveSupport::Concern

  included do
    rescue_from HTTPError do |e|
      render_error e.status_code, e.message
    end
  end

  class HTTPError < StandardError
    attr_accessor :status_code

    def initialize(status, message = nil)
      @status_code = Rack::Utils.status_code status
      super message || Rack::Utils::HTTP_STATUS_CODES[@status_code]
    end
  end

  def render_error(status, message = nil)
    status_code = Rack::Utils.status_code status
    message ||= Rack::Utils::HTTP_STATUS_CODES[status_code]
    body = {:status => status_code, :message => message}

    respond_to do |format|
      format.json { render :json => body, :status => status_code }
      format.xml { render :xml => body, :status => status_code }
    end
  end

  def error!(status, message = nil)
    raise HTTPError.new status, message
  end
end