go-tour: fix error highlighting when running locally
go-tour: fix error highlighting when running locally

Fixes issue 9119.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/177980043

Go-zh/tour-old,minux/go-tour-zh,minux/go-tour-zh,Go-zh/tour-old