HPCC-12729 Redesign memcached plugin exception handling

Signed-off-by: James Noss <james.noss@lexisnexis.com>
Merge pull request #6759 from jamienoss/issue12729-rethink-ALL-memcached-wu-exceptions-5.2.0

HPCC-12729 Redesign memcached plugin exception handling

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>