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.
The "already" isn't needed in this name, it doesn't add any meaning above "started".