scheduler: add Relay as a worker stage and fix bug

_utils/terror_gen/errors_release.txt

@@ -518,7 +518,7 @@ ErrSchedulerSourceOpRelayExist,[code=46023:class=scheduler:scope=internal:level=
 ErrSchedulerLatchInUse,[code=46024:class=scheduler:scope=internal:level=low], "Message: when %s, resource %s is in use by other client, Workaround: Please try again later"
 ErrSchedulerSourceCfgUpdate,[code=46025:class=scheduler:scope=internal:level=low], "Message: source can only update relay-log related parts for now"
 ErrSchedulerWrongWorkerInput,[code=46026:class=scheduler:scope=internal:level=medium], "Message: require DM master to modify worker [%s] with source [%s], but currently the worker is bound to source [%s]"
-ErrSchedulerCantTransferToRelayWorker,[code=46027:class=scheduler:scope=internal:level=medium], "Message: require DM worker to be bound to source [%s], but it has been started relay for source [%s]"
+ErrSchedulerBoundDiffWithStartedRelay,[code=46027:class=scheduler:scope=internal:level=medium], "Message: require DM worker [%s] to be bound to source [%s], but it has been started relay for source [%s], Workaround: If you intend to bind the source with worker, you can stop-relay for current source."
 ErrCtlGRPCCreateConn,[code=48001:class=dmctl:scope=internal:level=high], "Message: can not create grpc connection, Workaround: Please check your network connection."
 ErrCtlInvalidTLSCfg,[code=48002:class=dmctl:scope=internal:level=medium], "Message: invalid TLS config, Workaround: Please check the `ssl-ca`, `ssl-cert` and `ssl-key` config in command line."
 ErrCtlLoadTLSCfg,[code=48003:class=dmctl:scope=internal:level=high], "Message: can not load tls config, Workaround: Please ensure that the tls certificate is accessible on the node currently running dmctl."