go-tour: show "go.exe is not found" message
go-tour: show "go.exe is not found" message

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/6201066

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang-tr/go-tour-tr,golang-tr/go-tour-tr