HPCC-9792 Improve the error message on invalid use of STEPPED

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5737 from ghalliday/issue9792b

HPCC-9792 Improve the error message on invalid use of STEPPED

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>