Return 404 for missing documentation

Previously a 500 including a backtrace was returned which is hard to
read and understand.  Now a 404 response is returned, but at present
this is the WEBrick default 404 page.  A future commit is needed to fix
this.
Remove extra < in servlet 404 template
Make Servlet#not_found more generic

This will allow 404 messages with content other than a path.
Use nice 404 page for the server

This replaces the generic WEBrick 404 page with a rendered 404 page
matching the generator.
Move bug fixes to the correct release