Make it possible for a backend to disallow guest users

Reproduction steps

1. Connect to a backend connected to a private resource (e.g., a private GitHub repo, an XWiki instance with no guest access).
2. observe the landing page

Expected

The user is presented with a login action, with explanations

Actual

• The user has no particular explanations
• Technical message like " The requested page could not be found. You can edit the page to create it. " are presented to the user
• The new page action is proposed
• the edit button is visible and enabled