Improve error messages of FsiEvaluationSession

This helps debug situations similar to https://github.com/fsharp/FSharp.Compiler.Service/issues/199
There is a small rish that this "sprintf" may fail, raising a new exception.  Can you place a 

    try ... with _ -> ""

around the sprintf call please?

Also the sprintf call can be replaced by (err.ToString()), though still use an error handler.