Backport IdentityLinker#failed? from GroupSaml callback flow

app/controllers/omniauth_callbacks_controller.rb

@@ -82,7 +82,7 @@ class OmniauthCallbacksController < Devise::OmniauthCallbacksController
 
       if identity_linker.changed?
         redirect_identity_linked
-      elsif identity_linker.error_message.present?
+      elsif identity_linker.failed?
         redirect_identity_link_failed(identity_linker.error_message)
       else
         redirect_identity_exists

lib/gitlab/auth/omniauth_identity_linker_base.rb

@@ -17,6 +17,10 @@ module Gitlab
         @changed
       end
 
+      def failed?
+        error_message.present?
+      end
+
       def error_message
         identity.validate