HPCC-9199 Invalid params to control:querystats are not reported

Extend mergestats capabilities to include exceptions.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4338 from richardkchapman/mergestats_exception

HPCC-9199 Invalid params to control:querystats are not reported

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>