HPCC-11728 Avoid pointless exception in getLogicalFileSuperSubList

If a superfile disappears between gaining list of supers and
connecting to each, then skip instead of throwing an exception

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6176 from jakesmith/hpcc-11728

HPCC-11728 Avoid pointless exception in getLogicalFileSuperSubList

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>