I spent a number of hours recently trying to debug why mysqldump stopped
working. In reality it appears that the problem was borg-backup's ssh
connectivity causing the mysql-dump to not be able to stream properly.
Add a logging note about checking the other half of backup streaming
when the streaming script fails to try and make it more obvious this may
ocurr.
Change-Id: Idf53fc9e61f05077b954d730d88beab0cc1db09b
dda97505d8