[#746] morph: Add error return of MagicNumber method

Since morph `Client` works in multi-client mode, there is an error case when
we can not get network magic when all endpoints are unavailable.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/netmap.go

@@ -261,14 +261,19 @@ type netInfo struct {
 	netState netmap.State
 
 	magic interface {
-		MagicNumber() uint64
+		MagicNumber() (uint64, error)
 	}
 }
 
 func (n *netInfo) Dump() (*netmapV2.NetworkInfo, error) {
+	magic, err := n.magic.MagicNumber()
+	if err != nil {
+		return nil, err
+	}
+
 	ni := new(netmapV2.NetworkInfo)
 	ni.SetCurrentEpoch(n.netState.CurrentEpoch())
-	ni.SetMagicNumber(n.magic.MagicNumber())
+	ni.SetMagicNumber(magic)
 
 	return ni, nil
 }

pkg/morph/client/client.go

@@ -390,22 +390,15 @@ func toStackParameter(value interface{}) (sc.Parameter, error) {
 // to which the underlying RPC node client is connected.
 //
 // Returns 0 in case of connection problems.
-func (c *Client) MagicNumber() (res uint64) {
+func (c *Client) MagicNumber() (res uint64, err error) {
 	if c.multiClient != nil {
-		err := c.multiClient.iterateClients(func(c *Client) error {
-			res = c.MagicNumber()
-			return nil
+		return res, c.multiClient.iterateClients(func(c *Client) error {
+			res, err = c.MagicNumber()
+			return err
 		})
-		if err != nil {
-			c.logger.Debug("iterate over client failure",
-				zap.String("error", err.Error()),
-			)
-		}
-
-		return
 	}
 
-	return uint64(c.client.GetNetwork())
+	return uint64(c.client.GetNetwork()), nil
 }
 
 // BlockCount returns block count of the network