whisper/mailserver: pass init error to the caller (#16671)

* whisper/mailserver: pass init error to the caller

* whisper/mailserver: add returns to fmt.Errorf

* whisper/mailserver: check err in mailserver init test

cmd/wnode/main.go

@@ -271,7 +271,9 @@ func initialize() {
 
 	if *mailServerMode {
 		shh.RegisterServer(&mailServer)
-		mailServer.Init(shh, *argDBPath, msPassword, *argServerPoW)
+		if err := mailServer.Init(shh, *argDBPath, msPassword, *argServerPoW); err != nil {
+			utils.Fatalf("Failed to init MailServer: %s", err)
+		}
 	}
 
 	server = &p2p.Server{