Add verbose error page. Change yhs_html_text a bit.

(In practice, the old defaults for yhs_html were rubbish.)
Add const... and why not.
Send any path to a handler if the prefix matches.

Previously handlers were only called for files. e.g., a handler for
"/a/" would be called for "/a/b" but not "/a/b/c". This is obviously
(or at least it is now) no good for the file server handler.
Fix file server handler; it was forming the path wrongly.