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.
Yeah, it's more of a sanity check and a test for future code changes (given how complicated some of the TD* code can be). But it should definitely be a test and not dealt with here.