feat: support exiting early from IterateHierarchy method (argoproj#388)

* feat: support existing early from IterateHierarchy method

Signed-off-by: Alexander Matyushentsev <AMatyushentsev@gmail.com>

* reviewer notes: comment action callback return value; add missing return value check

Signed-off-by: Alexander Matyushentsev <AMatyushentsev@gmail.com>