tapchannel: add extra logging when we cancel incoming invoice HTLCs

In this commit, we add some extra logging for the case where we cancel
incoming invoice HTLCs. This should make it easier to see exactly what's
going on re the why/when here.

Author: Roasbeef

tapchannel/aux_invoice_manager.go

@@ -191,7 +191,12 @@ func (s *AuxInvoiceManager) handleInvoiceAccept(ctx context.Context,
 		//
 		// TODO(george): Strict-forwarding could be configurable?
 		if isAssetInvoice(req.Invoice, s) {
+			iLog.Debugf("has no asset custom records, but " +
+				"invoice requires assets, canceling HTLCs")
+
 			resp.CancelSet = true
+		} else {
+			iLog.Tracef("has no asset custom records, ignoring")
 		}
 
 		return resp, nil
@@ -203,6 +208,9 @@ func (s *AuxInvoiceManager) handleInvoiceAccept(ctx context.Context,
 		// Since we requested btc we should be receiving btc.
 		resp.CancelSet = true
 
+		iLog.Debugf("has asset custom records, but invoice does not " +
+			"require assets, canceling HTLCs")
+
 		return resp, nil
 
 	default: