cbbackupmgr: Rollback on DCP disconnect is incorrect

Problem
The rollback logic on DCP disconnect is wrong. It incorrectly reports that the backup was successful and does not retry

Expectations
One of the following should happen:

1. An error message should be produce saying a roll back was received
2. Correctly roll back and restart the DCP stream from the beginning (This might not be a good idea because it possible the DCP stream gets disconnected again)
3. Be able to resume an a DCP stream that is behind the purge seqno, assuming the purge seqno has not changed. (This might not be possible/need changes from KV_engine)