Add row number and column name when an exception occurs.

I needed to see where a cast error was occurring, so I quickly added this information as Data fields on the thrown Exception.  Retrieve the data when an Exception occurs by catching the Exception and then looking at the `ex.Data["LinqToExcel.RowNumber"]` and `ex.Data["LinqToExcel.ColumnName"]` properties.

Noticed that someone else wanted it too (Issue #61), so here's a pull request.