Provide a means to do a clean abort

[hstenn]

Mostly, but not fully thought-out.

If the script could check at the top of the "what host next" loop for the existence of a well-known file, and send an email and alert that ghettoVCB was aborting because of the existence of the well-known file (and then exit), this might be a simple and clean way to stop ghettoVCB from doing anything if the file was created before the cron job runs, or if the cron job is already running, it will stop as soon as the current host backup is complete. Doing this would mean there's no need to delete ghettoVCB snapshots or do any consolidation.