tools/reaper: fatal when resources are not deleted

[darkowlzz]

When resources are found but not deleted, fail to help detect that stale resources were found but not deleted.

$ go run ./ -provider gcp -tags 'environment=dev' -gcpproject test-project
Total resources found: 11
artifactregistry.googleapis.com/Repository: projects/test-project/locations/us-central1/repositories/flux-test-sharing-sawfly
compute.googleapis.com/Instance: gke-flux-test-sharing-sa-default-pool-9ff69617-gk5w
compute.googleapis.com/Disk: gke-flux-test-sharing-sa-default-pool-9ff69617-gk5w
compute.googleapis.com/Instance: gke-flux-test-sharing-sa-default-pool-314d00bb-l1k6
compute.googleapis.com/Disk: gke-flux-test-sharing-sa-default-pool-314d00bb-l1k6
compute.googleapis.com/Disk: gke-flux-test-sharing-sa-default-pool-310d55fd-xjd5
compute.googleapis.com/Instance: gke-flux-test-sharing-sa-default-pool-310d55fd-xjd5
compute.googleapis.com/InstanceTemplate: gke-flux-test-sharing-sa-default-pool-310d55fd
compute.googleapis.com/InstanceTemplate: gke-flux-test-sharing-sa-default-pool-314d00bb
compute.googleapis.com/InstanceTemplate: gke-flux-test-sharing-sa-default-pool-9ff69617
container.googleapis.com/Cluster: flux-test-sharing-sawfly
2023/11/04 00:49:45 resources found but not deleted
exit status 1

Related to the incident described in fluxcd/pkg#682, to be able to detect such situations easily in the future.