Fix VolumeSnapshot deletion not retrying when used for PVC restore

[Paramesh324]

What type of PR is this?
/kind bug

What this PR does / why we need it:
When a VolumeSnapshot is deleted while a PVC is still being created from it, the controller blocked deletion but returned nil, so the workqueue never retried. This change returns an error instead so the snapshot is requeued and deletion is retried once the PVC is no longer in use.

The same fix is applied for VolumeGroupSnapshot in the group snapshot helper.

Which issue(s) this PR fixes:
Fixes #1366

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fixed VolumeSnapshot (and VolumeGroupSnapshot) deletion getting stuck when the snapshot was in use for a PVC restore. The controller now requeues and retries deletion until the PVC is no longer in use.```

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• ✅ login: Paramesh324 / name: Parameshwaran Krishnasamy (3fc8f02)

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This pull-request has been approved by: Paramesh324
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