devise/app/controllers/devise/omniauth_callbacks_controller.rb

class Devise::OmniauthCallbacksController < ApplicationController
  include Devise::Controllers::InternalHelpers

  def failure
    set_flash_message :alert, :failure, :kind => failed_strategy.name.to_s.humanize, :reason => failure_message
    redirect_to after_omniauth_failure_path_for(resource_name)
  end

  protected

  def failed_strategy
    env["omniauth.failed_strategy"]
  end

  def failure_message
    exception = env["omniauth.error"]
    error   = exception.error_reason if exception.respond_to?(:error_reason)
    error ||= exception.error        if exception.respond_to?(:error)
    error ||= env["omniauth.failure_key"]
    error.to_s.humanize if error
  end

  def after_omniauth_failure_path_for(scope)
    new_session_path(scope)
  end
end