ArcRepository should handle failing checksum jobs

When receiving the reply of a requested checksum
(in ArcRepository.processChecksum()) it should be
checked whtether the received message is OK.
If it is not this signifies some sort of error
on the bitarchive.
At the moment this is not checked. If the message is
not OK the code may throw some mysterious Exception
or perhaps even progress and do damage.
NOTE: This bug is originally from Bugzilla bug_id=334.
This bug was originally submitted by NHC.