Make the error message and the needed action more obvious when a command isn't found to obtain the source code of a project. Users seem to strugle with the existing wording in practice. R=rsc CC=golang-dev https://golang.org/cl/4058047
R=rsc CC=golang-dev https://golang.org/cl/3448042
Tiny helper to avoid strings.Index(s, sub) != -1 R=rsc, r2, r CC=golang-dev https://golang.org/cl/2265044
R=brainman, rsc, vcc, rsc1 CC=golang-dev https://golang.org/cl/2068041