Log non persisted users

app/controllers/users/omniauth_callbacks_controller.rb

@@ -16,6 +16,7 @@ class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
       redirect_to user_obtain_token_path, :event => :authentication, :current_user => @user
     else
       session["devise.facebook_data"] = request.env["omniauth.auth"]
+      Rails.logger.error("Failed to persist user: #{@user.errors.full_messages.join(', ')}")
       #TAG: facebook
       # redirect_to new_user_registration_url
       redirect_to '/'