TOOLS-970: 'sdcadm experimental update-gz-tools ...' failed on 'hist.error'
TOOLS-970: 'sdcadm experimental update-gz-tools ...' failed on 'hist.error'

Also catches a case where the "justDownload" code path wouldn't report
any errors that occurred.

joyent/sdcadm,joyent/sdcadm,joyent/sdcadm