Support 404.html

[kiootic]

• If file not found, show 404.html
  • /index.html not found, try /404.html
  • /foo/index.html not found, try /foo/404.html & /404.html
• Status code keep 404.

[toshinari123]

• create error_handler.go in internal/handler/site, which has an handleError func
• handleError is recursive as follows:
  • serveFile will call handleError if not found, then it will call back serveFile to serve 404
  • if that serveFile call is not found, it will call handleError which goes up a layer in the path

write handleError [2 hours]
test handleError by adding 404 in root of example and adding subdirectoies [2 hours]
write error_handler_test.go [4 hours]

[kiootic]

• Please make it a middleware; I'd expect it to rewrite the URL path into 404.html if the requested path does not exists, and pass the updated URL to the site handler.
• Please avoid unneeded recursive function; it should be easy to implement the logic without recursion in this case.