Backport: Replace the FATAL_ERROR in dune_policy(SET) by an author warning

This MR backports some changes from !1511 (merged) such that undefined policies do not trigger a fatal error when dune_policy(SET) is called.

added backport buildsystem labels

requested review from @santiago.ospina

assigned to @simon.praetorius

approved this merge request

enabled an automatic merge when all merge checks for 40a0db57 pass

Looks good to me, thanks!

Ok, the CI for the other core modules is not really prepared to handle a target branch other than master (this targets releases/2.10).

canceled the automatic merge

mentioned in commit 67db40f2

merged