cmd/go: add missing error check
cmd/go: add missing error check

R=golang-dev, minux.ma, bradfitz
CC=golang-dev
http://codereview.appspot.com/5874055

Committer: Shenghou Ma <b563c57947dc20eb81988e3e3c22786764c14ca8@gmail.com>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go