services: improve notary service logging

Let the user know whether it's main or fallback transaction that failed
to be relayed to the network.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

pkg/services/notary/notary.go

@@ -480,7 +480,9 @@ func (n *Notary) newTxCallbackLoop() {
 			n.reqMtx.Unlock()
 			err := n.onTransaction(tx.tx)
 			if err != nil {
-				n.Config.Log.Error("new transaction callback finished with error", zap.Error(err))
+				n.Config.Log.Error("new transaction callback finished with error",
+					zap.Error(err),
+					zap.Bool("is main", isMain))
 				continue
 			}