autobuild: Give a clearer failure message

autobuild: Give a clearer failure message

This helps when autobuild.py is used in --tail mode and
where there is neither e-mail nor access to the logs.tar.gz

Working back to find where the error happened is typically
quite difficult, as many failures are actually due to the
cleanup.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>