Fix when exporting to excel files

When exporting to excel, there cannot be more than 255 columns.
If there are more columns than that, we write "ERROR: TOO MANY
COLUMNS" to the 255th column. Formerly, OpenRefine reported
a 500 Server error.
Merge pull request #867 from abhillman/exceloutput255bugfix

Report error to user when attempting to export >255 columns, rather than generic 500 ISE