Force the Source mode when changing the document syntax, if the new syntax doesn't have a renderer

We don't load the WYSIWYG editor if the document syntax doesn't have a renderer, but the user can change the syntax after the editor is loaded. It would be nice to force the Source mode in this case with a warning message for the user.

This can be reproduced by installing the MediaWiki syntax and changing the document syntax while editing with the WYSIWYG editor.