clean.bash: stop if $GOROOT is not set
clean.bash: stop if $GOROOT is not set

Doing rm -rf /pkg/.. blindly isn't nice.  It could have
unintended consequences.

Secondly set bash to abort on (unexpected) errors.

R=dho, rsc
CC=golang-dev
http://codereview.appspot.com/176056

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go