cmd/go-contrib-init: better GOPATH checkout error message

Modify the error message to provide more context if the user is in the
wrong directory. (The default Terminal.app does not do a great job of
showing what directory you are currently in).

Change-Id: Iafcbbd2070ca968863120fb8c4bc15609475cda2
Reviewed-on: https://go-review.googlesource.com/48232
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed