fix: return backup lookup errors when deleting snapshots

[immanuwell]

Fixes a path where a Cinder backup lookup failure is ignored during DeleteSnapshot.

A non-404 error now reaches the snapshotter, so it retries instead of treating a later snapshot 404 as success.

Repro:

1. Use a VolumeSnapshotClass with type: backup
2. Make Cinder return 500 or 503 to GET /backups/<id>
3. Delete the VolumeSnapshot. Before this, it can report success and leave the backup behind.

Tests: make unit, go test ./tests/sanity/cinder

Release note:

[cinder-csi-plugin] Return Cinder backup lookup failures when deleting a backup-backed VolumeSnapshot so the snapshotter retries deletion.

[kubernetes-prow]

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[kubernetes-prow]

Hi @immanuwell. Thanks for your PR.

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[kayrus]

/ok-to-test

@immanuwell please set a proper release note in the initial PR message

[immanuwell]

@kayrus addressed your suggestions, PTAL

[kayrus]

/test pull-cloud-provider-openstack-check