go-tour: add log.Fatal to HTTP handlers exercise

This helps people diagnose the ListenAndServe error.

TBR=campoy
R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/115170043
diff --git a/content/prog/tour/exercise-http-handlers.go b/content/prog/tour/exercise-http-handlers.go
index 57dc8bf..a824531 100644
--- a/content/prog/tour/exercise-http-handlers.go
+++ b/content/prog/tour/exercise-http-handlers.go
@@ -3,10 +3,11 @@
 package main
 
 import (
+	"log"
 	"net/http"
 )
 
 func main() {
 	// your http.Handle calls here
-	http.ListenAndServe("localhost:4000", nil)
+	log.Fatal(http.ListenAndServe("localhost:4000", nil))
 }