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.
It seems like this should be caught in tests not runtime -- is it ever possible to create the wrong type of replicator during createReplicatorWithProperties:, if so it feels like a bug on our side?