update 404 exception route in web to render 404 error template.

The original 404 page was a self-contained HTML template. This meant it didn't
use the render function like the rest of the routes in the web package. I had to
change merge-pathnames to render because I wanted the 404 page to use the Djula
tempting engine. By doing this, the 404 page could use the main.css file and the
insert-snippet lisp code-block.

src/web.lisp

@@ -1856,5 +1856,4 @@
 
 (defmethod on-exception ((app <web>) (code (eql 404)))
   (declare (ignore app))
-  (merge-pathnames #P"_errors/404.html"
-                   *template-directory*))
+  (render #P"_errors/404.html"))