ci: Require a missing file in review-cleanup

What does this MR do and why?

This fixes a problem that I noticed in the latest scheduled pipelines: https://gitlab.com/gitlab-org/gitlab/-/jobs/1882193040

review-cleanup shows an error but continues when the namespace to clean up cannot be found:

$ delete_release
** Checking if review-fix-review-yclzl4 exists in the review-fix-review-yclzl4 namespace... **
Previous deployment for review-fix-review-yclzl4 NOT found.
$ delete_namespace
** Checking if review-fix-review-yclzl4 exists... **
Namespace review-fix-review-yclzl4 NOT found.

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