HPCC-8878 Improve the error message if MAXLENGTH is too small

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4022 from ghalliday/issue8878

HPCC-8878 Improve the error message if MAXLENGTH is too small

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>