Log 404s generated by the explicit 404 handler

Other 404s are already logged in their respective handlers.

main.go

@@ -155,7 +155,9 @@ func main() {
 		os.Exit(1)
 	}
 
-	http.HandleFunc("/favicon.ico", http.NotFound)
+	logged404 := log.Request(http.HandlerFunc(http.NotFound), kitlog.With(logger, "code", 404))
+
+	http.Handle("/favicon.ico", logged404)
 
 	http.Handle("/static/", log.Request(addCacheHeaders(http.FileServer(http.FS(staticFS))), logger))
 
@@ -180,7 +182,7 @@ func main() {
 
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.Path != "/" {
-			http.NotFound(w, r)
+			logged404.ServeHTTP(w, r)
 			return
 		}