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

whisper/mailserver/server_test.go

@@ -92,7 +92,10 @@ func TestMailServer(t *testing.T) {
 	shh = whisper.New(&whisper.DefaultConfig)
 	shh.RegisterServer(&server)
 
-	server.Init(shh, dir, password, powRequirement)
+	err = server.Init(shh, dir, password, powRequirement)
+	if err != nil {
+		t.Fatal(err)
+	}
 	defer server.Close()
 
 	keyID, err = shh.AddSymKeyFromPassword(password)