Dump: make labs-rsync-cron.sh paranoid

Check that the destination mount (/mnt/dumps) is there before
trying the copy and bail out if it doesn't seem to be there.

Change-Id: I3b69bf5c87d29e94c947ccd6125592ba426f1c8c
Merge "Dump: make labs-rsync-cron.sh paranoid" into production