HPCC-12002 Clarify SegMonitorList::checkSize error message

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6294 from jakesmith/hpcc-12002

HPCC-12002 Clarify SegMonitorList::checkSize error message

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>