This is actually documented, albeit maybe a bit hard to find:  http://weitz.de/hunchentoot/#acceptor-status-message  -  I'd gladly accept a documentation patch that puts some prose describing how to customize error pages into a separate chapter.

With no further customization, the error page templates are located in the www/errors/ subdirectory of the Hunchentoot source distribution.

Let me know if you have further questions.

-Hans