HPCC-9947 Improve the warning for an invalid use of NOTHOR

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4998 from ghalliday/issue9947

HPCC-9947 Improve the warning for an invalid use of NOTHOR

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>