cmd/go: set exit status for failing "go generate" run.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/154360048
flag: roll back 156390043 (flag setting)
Shell scripts depend on the old behavior too often.
It's too late to make this change.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/161890044