- Close iterators if newItrCombiner fails. - Handle nil iterators returned without errors. - Improve error messages for debugging. Signed-off-by: David Enyeart <enyeart@us.ibm.com> (cherry picked from commit 9bcca1ad011c97dbc2731102ce74b361d6b5c5a1) Co-authored-by: David Enyeart <enyeart@us.ibm.com>