server: don't mention aux stuff in error [skip ci]

This error is returned when one attempts to use taproot overlay channels
without the necessary auxiliary funding controller being supplied. In
practice, when running lnd via litd, the latter will always supply
'tapd' as the aux funding controller, but a standalone lnd has no
knowledge of that.

The previous message sort of reads like the lnd user has forgotten to
set some available option or flag, but nothing or the sort actually
exists. The update makes it clearer that the feature isn't available in
the standalone build.

server.go

@@ -634,8 +634,9 @@ func newServer(ctx context.Context, cfg *Config, listenAddrs []net.Addr,
 	if cfg.ProtocolOptions.TaprootOverlayChans &&
 		implCfg.AuxFundingController.IsNone() {
 
-		return nil, fmt.Errorf("taproot overlay flag set, but not " +
-			"aux controllers")
+		return nil, fmt.Errorf("taproot overlay flag set, but " +
+			"overlay channels are not supported " +
+			"in a standalone lnd build")
 	}
 
 	//nolint:ll