Renderer graceful fallback

Resolves [FS#1767](https://bugs.dokuwiki.org/index.php?do=details&task_id=1767)

After implementing this, I wonder if fallback should only be applied when $mode == 'xhtml'.  The problem it solves is a replacement renderer plugin being disabled/uninstalled when it is the configured xhtml renderer and effectively all but disabling the wiki.