Don't break campaign resolver when a related user is deleted
Created by: eseliger
We allow related users to be deleted, so we should allow to still return without error. This adds the check on the err and adds tests.
Also, I've removed the "reset error when namespace is not found", because namespace
is not nullable (and shouldn't be, because it's unreachable when no namespace exists anymore). Hence, the API failed with an ambiguous error message "unexpected null returned for namespace", instead of "cannot find user ...".