#2568: make error message when using incorrect reduction handle more informative

Comment (by Roland Haas):

During the ET call on 2021-09-30 (http://lists.einsteintoolkit.org/pipermail/users/2021-September/008219.html) Yosef looked at the pull request and spotted a type but was otherwise fine. I will take this is a “please apply”.

Applied as git hash 3842b076 "Cactus: add explicit error mesage if wrong reductin op handles are used" of cactus