Editing an unsupported syntax is not supported well

Reproduction steps

1. create a page with syntax xwiki/2.1 (using xwiki)
2. go to cristal and edit the page

Expected

• Editing is not proposed, with an explanation of why (i.e., the syntax is not editable)
• In case of direct access (e.g., by typing the right url manually), a nice translated message is presented to the user, and an action is available is go back to view

Actual

• editing is always proposed if the current user has edit rights
• in case of direct access, the user is stuck and have to navigate to another page though the navigation tree to escape