go/build: remove 'go/build' from error messages
go/build: remove 'go/build' from error messages

This leads to really confusing messages in goinstall.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5495074

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go