[#521] Fix issues with transition from pkg/errors pkg

Wrap functions at `pkg/errors` return nil if error argument
was nil. fmt.Errorf always returns error so we need to add
missing error checks to the code.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

cmd/neofs-node/netmap.go

@@ -111,8 +111,10 @@ func bootstrapNode(c *cfg) {
 	initState(c)
 
 	err := c.cfgNetmap.wrapper.AddPeer(c.toOnlineLocalNodeInfo())
+	if err != nil {
 		fatalOnErr(fmt.Errorf("bootstrap error: %w", err))
 	}
+}
 
 func addNetmapNotificationHandler(c *cfg, sTyp string, h event.Handler) {
 	typ := event.TypeFromString(sTyp)
@@ -136,10 +138,14 @@ func setNetmapNotificationParser(c *cfg, sTyp string, p event.Parser) {
 
 func initState(c *cfg) {
 	epoch, err := c.cfgNetmap.wrapper.Epoch()
+	if err != nil {
 		fatalOnErr(fmt.Errorf("could not initialize current epoch number: %w", err))
+	}
 
 	ni, err := c.netmapLocalNodeState(epoch)
+	if err != nil {
 		fatalOnErr(fmt.Errorf("could not init network state: %w", err))
+	}
 
 	c.handleNodeInfoStatus(ni)
 

pkg/core/object/fmt.go

@@ -81,7 +81,7 @@ func (v *FormatValidator) Validate(obj *Object) error {
 
 		// TODO: combine small checks
 		if err := v.checkExpiration(obj); err != nil {
-			return fmt.Errorf("object did not pass expiration ch: %weck", err)
+			return fmt.Errorf("object did not pass expiration check: %w", err)
 		}
 
 		if err := object.CheckHeaderVerificationFields(obj.SDK()); err != nil {

pkg/services/object/put/streamer.go

@@ -157,11 +157,13 @@ func (p *Streamer) SendChunk(prm *PutChunkPrm) error {
 		return errNotInit
 	}
 
-	_, err := p.target.Write(prm.chunk)
+	if _, err := p.target.Write(prm.chunk); err != nil {
-
 		return fmt.Errorf("(%T) could not write payload chunk to target: %w", p, err)
 	}
 
+	return nil
+}
+
 func (p *Streamer) Close() (*PutResponse, error) {
 	if p.target == nil {
 		return nil, errNotInit