make error message when using incorrect reduction handle more informative

[Roland Haas]

There is ongoing confusion about which reduction operator handle to use with the reduction interface offered by the main driver Carpet.

Pull request https://bitbucket.org/cactuscode/cactus/pull-requests/128/cactus-add-explicit-error-mesage-if-wrong adds some extra help text to the warning message produced by Cactus (it’s a level 3 warning and non-zero return code just like passing an invalid handle to the routine).

[Roland Haas]

Please review.

[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