HPCC-10765 Better error report for index filename matching dataset field

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6552 from ghalliday/issue10765

HPCC-10765 Better error report for index filename matching dataset field

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>