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

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).