Changes the CDTReplicator -start signature.

-(void)start becomes -(BOOL)start:NSError**

Errors created by TDReplicator objects are propaged from
TDReplicatorManager. They are reported to the Log (via Warn),
but new CDT-level errors are reported to developers using the
CDT API.
All these `if` statements seem to be calling STFail if the replication starts successfully. Why is this?