void OnGroupChatMessageError(Message msg, XmppGroupChatModel chat)
{
var builder = CreateMessageBuilder();
// TODO: nicer formatting
if (msg.Error.ErrorText != null) {
builder.AppendErrorText(msg.Error.ErrorText);
} else {
builder.AppendErrorText(msg.Error.ToString());
}
Session.AddMessageToChat(chat, builder.ToMessage());
}