[x/tour] go-tour: add missing error/rune predeclared identifier for Syntax-Highlighting
[x/tour] go-tour: add missing error/rune predeclared identifier for Syntax-Highlighting

R=golang-dev, khr, adg
CC=golang-dev
https://golang.org/cl/11034043
X-Tour-Commit: dd74111beef6622228443a9916ba45cead9488d2

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website