improve error page for authenticatedSubjectNotFound condition when external registration is also enabled

Example v2.2 URL: grouperExternal/public/UiV2Public.index?operation=UiV2Public.postIndex&function=UiV2Public.error&code=authenticatedSubjectNotFound

It would likely be helpful if the page could include a "Hey, maybe you need to register HERE(link) first?" message. 

Suggestion:

  Add the ability to include the link on the page.

  Maybe just conditional on if external registration is configured,

  Maybe on some other "opt in" config option. (like setting the URL to include.) etc..