HPCC-11059 - Pass subscription exception back to client

Invalid subscriptions failed silently.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5560 from jakesmith/hpcc-11059

HPCC-11059 - Pass subscription exception back to client

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>