HPCC-10251 Fix windows build break

Call MakeStringException instead.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5021 from ghalliday/issue10251

HPCC-10251 Fix windows build break

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>