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

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/154360048

sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc