[master] Fix infinite loop in execute() of BKWatchAgent

There should be no return from within the execute() method of the BookkeepingWatchAgent... Replaced them with continue

A few improvements in dirac-dms-check-fc2se were also ready in my local installation and are coming with it... Just a check in SE when replicas are not registered in the FC and possibility to register the missing replica. This is a continuation of the previous improvement to register a file that is not in the FC.