Add error if no API user found

app/controllers/registrar/tara_controller.rb

@@ -2,19 +2,27 @@ class Registrar
   class TaraController < ApplicationController
     skip_authorization_check
 
+    # rubocop:disable Style/AndOr
     def callback
       session[:omniauth_hash] = user_hash
       @api_user = ApiUser.from_omniauth(user_hash)
 
-      return unless @api_user
-
+      if @api_user
         sign_in_and_redirect(:registrar_user, @api_user)
+      else
+        show_error and return
       end
+    end
+    # rubocop:enable Style/AndOr
 
     def cancel
       redirect_to root_path, notice: t(:sign_in_cancelled)
     end
 
+    def show_error
+      redirect_to new_registrar_user_session_url, alert: t(:no_such_user)
+    end
+
     private
 
     def user_hash