ensure xymon dot is red if we have a backup task that has never completed

We had been raising a failure report if we had never seen a succesful backup
for a host. However, when we have a host with more than one backup task, we
can have the situation where one backup is working OK but the other has never
completed correctly. This let to a green report as we had a non-zero number
of rows, but we require number_of_good_backups == number_of_tasks !