cbbackupmgr backup can complete sucessfully whilst ignoring errors from failed vBuckets (Errors are logged)

There a two cases that can be easily be explained:
1) On receiving 5 ErrNetwork errors when trying to open a stream cbbackupmgr will mark that vBucket as complete and continue. This means cbbackupmgr will complete "successfully" and inform the user of that, however, 'n' vBuckets may have silently failed to stream.
2) On receiving a single ErrShutdown cbbackupmgr will mark the vBucket as completed and continue. The outcome is the same as the scenario above.

In both scenarios, the errors (and the fact that the vBuckets were skipped) will be included in the logs.