HPCC-10537 Generate a warning if MERGEJOIN is not specified as LOCAL

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-10537 Make it easier to report warnings with error severities

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6644 from ghalliday/issue10537

HPCC-10537 Generate a warning if MERGEJOIN is not specified as LOCAL

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>