Have broctl return useful exit codes

I've got a broctl branch here: https://github.com/grigorescu/broctl which aims to have it return a 0 or 1 exit code for most execution paths. My dive down this particular rabbit hole started when I wanted to have status return a non-zero exit code if a node had failed, but I tried to cover everything else while I was at it.

If someone could double-check it, to make sure that I didn't miss anything, it'd be much appreciated.