[#973] node: Resolve perfsprint linter

`fmt.Errorf can be replaced with errors.New` and `fmt.Sprintf can be replaced with string addition`

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/util/sdnotify/sdnotify.go

@@ -1,6 +1,7 @@
 package sdnotify
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"os"
@@ -13,14 +14,19 @@ const (
 	ReloadingEnabled = "RELOADING=1"
 )
 
-var socket *net.UnixAddr
+var (
+	socket *net.UnixAddr
+
+	errSocketVariableIsNotPresent = errors.New("\"NOTIFY_SOCKET\" environment variable is not present")
+	errSocketIsNotInitialized     = errors.New("socket is not initialized")
+)
 
 // Initializes socket with provided name of
 // environment variable.
 func InitSocket() error {
 	notifySocket := os.Getenv("NOTIFY_SOCKET")
 	if notifySocket == "" {
-		return fmt.Errorf("\"NOTIFY_SOCKET\" environment variable is not present")
+		return errSocketVariableIsNotPresent
 	}
 	socket = &net.UnixAddr{
 		Name: notifySocket,
@@ -38,14 +44,14 @@ func FlagAndStatus(status string) error {
 
 // Status sends systemd notify STATUS=%s{status}.
 func Status(status string) error {
-	return Send(fmt.Sprintf("STATUS=%s", status))
+	return Send("STATUS=" + status)
 }
 
 // Send state through the notify socket if any.
 // If the notify socket was not detected, it returns an error.
 func Send(state string) error {
 	if socket == nil {
-		return fmt.Errorf("socket is not initialized")
+		return errSocketIsNotInitialized
 	}
 	conn, err := net.DialUnix(socket.Net, nil, socket)
 	if err != nil {