In this case, `data.Value` will contain the error message returned from the server. So I would push an error message like this:

    errMsg = "Expression " + (idx + 1) + ": " + data.Value;

The reason for not including the expression itself in the error messages: we have some *huge* expressions. Also, we have some cases where we have multiple times the same expression from different servers in one graph. So the expression ID is the best way to identify where an error comes from.