main.go

package main

import (
	"fmt"
	"log"
	"net/http"
)

func main() {
	handler := NotFoundHandler(http.FileServer(http.Dir(".")), http.HandlerFunc(Custom404))
	log.Fatal(http.ListenAndServe(":8080", handler))
}

func NotFoundHandler(h http.Handler, n http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		writer := &NotFoundResponseWriter{w, 0}
		h.ServeHTTP(writer, r)
		if writer.status == http.StatusNotFound {
			n.ServeHTTP(w, r)
		}
	})
}

type NotFoundResponseWriter struct {
	http.ResponseWriter
	status int
}

func (n *NotFoundResponseWriter) WriteHeader(status int) {
	n.status = status
	n.ResponseWriter.WriteHeader(status)
}

func (n *NotFoundResponseWriter) Write(p []byte) (int, error) {
	if n.status == http.StatusNotFound {
		return len(p), nil
	}
	return n.ResponseWriter.Write(p)
}

func Custom404(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintf(w, "%s: Not found", r.URL)
}

check out this comment:
https://gist.github.com/ciaranarcher/abccf50cb37645ca27fa#gistcomment-1908904