How to handle exceptions like 401, 501, 404 in Rails

application_controller.rb

# As used with CanCan and Devise
class ApplicationController < ActionController::Base
  protect_from_forgery

  include ErrorResponseActions

  rescue_from CanCan::AccessDenied, :with => :authorization_error
  rescue_from ActiveRecord::RecordNotFound, :with => :resource_not_found

  before_filter :authenticate!

  private

  def authenticate!
    current_user || devise_controller? || ( authentication_error and return false )
  end
end

error_response_actions.rb

module ErrorResponseActions

  ERROR_RESPONSE_ACTIONS = %[authentication_error
                             authorization_error
                             resource_not_found
                             page_not_found
                             not_implmented
                             route_not_found
                             method_not_allowed].freeze

  def authentication_error
    respond_to do |format|
      format.html do
        flash[:error] = 'You are not logged in.'
        session[:user_return_to] = request.fullpath
        redirect_to new_user_session_path
      end
      # 401 Unauthorized response
      format.any(:json, :xml){ request_http_basic_authentication 'My Application' }
    end
  end

  def authorization_error
    # 403 Forbidden response
    respond_to do |format|
      format.html{ render '/rescues/access_denied', :status => 403 }
      format.xml{  render :xml => 'Access Denied',  :status => 403 }
      format.json{ render :json => 'Access Denied', :status => 403 }
    end
  end

  def resource_not_found
    respond_to do |format|
      format.html{ render :template=>'/rescues/record_not_found', :status => 404 }
      format.xml{  render :xml => 'Record Not Found',             :status => 404 }
      format.json{ render :json => 'Record Not Found',            :status => 404 }
    end
  end

  def page_not_found
    respond_to do |format|
      format.html{ render :template=>'/rescues/page_not_found', :status => 404 }
      format.xml{  render :xml => 'Page Not Found',             :status => 404 }
      format.json{ render :json => 'Page Not Found',            :status => 404 }
    end
  end

  def not_implmented(allowed_methods)
    method_not_allowed(allowed_methods, 'Not Implemented', 501)
  end

  def method_not_allowed(allowed_methods, message = 'Method Not Allowed', status=405)
    response.headers['Allow'] ||= allowed_methods.map { |method_symbol| method_symbol.to_s.upcase } * ', '
    respond_to do |format|
      format.html{ render :template=>'/rescues/method_not_allowed', :status => status }
      format.xml{  render :xml => message,                          :status => status }
      format.json{ render :json => message,                         :status => status }
    end
  end

  def route_not_found
    method, path = env['REQUEST_METHOD'].downcase.to_sym, env['PATH_INFO']

    # Route was not recognized. Try to find out why (maybe wrong verb).
    allows = ActionDispatch::Routing::HTTP_METHODS.select { |verb|
      begin
        match = Rails.application.routes.recognize_path(path, :method => verb)
        match[:action] != 'route_not_found'
      rescue ActionController::RoutingError
        nil
      end
    }

    if !allows.empty? && !ActionDispatch::Routing::HTTP_METHODS.include?(method)
      not_implmented allows
      # raise ActionController::NotImplemented.new(*allows)
    elsif !allows.empty?
      method_not_allowed allows
      # raise ActionController::MethodNotAllowed.new(*allows)
    else
      if Rails.configuration.consider_all_requests_local
        response.headers['X-Cascade'] = 'pass'
        render :nothing => true, :status => 404
      else
        page_not_found
      end
    end
  end

end

routes.rb

Rails.application.routes.draw do

  # Catch-all to allow proper HTTP responses for 405 and 501
  match "*route_not_found.:format", :to => "application#route_not_found"
  match "*route_not_found",         :to => "application#route_not_found"

end