Fallback to the wiki editor when there's no Renderer for a given syntax

Right now when a simple user tries to edit a page written in Markdown (for example) we get the attached screenshot. It would be better to do the following:

• display a warning that we cannot display the WYSIWYG editor (as it is now)
• display the wiki editor instead

Otherwise the user has to understand the concept of simple/advanced user and users don't know about this (I just tested it with a real user ).