[cinder-csi-plugin] Fix panic when backup lookup fails in CreateSnapshot

[immanuwell]

What this PR does / why we need it:

This fixes a nil pointer panic in the cinder CSI backup snapshot path.
If CreateSnapshot runs with snapshotType=backup, ListBackups finds one match, and GetBackupByID then errors, the code dereferences backup.ID and blows up. Kinda nasty.

The fix logs the already known backup ID from ListBackups and returns codes.Internal cleanly.

Which issue this PR fixes(if applicable):

fixes #

Special notes for reviewers:

Repro before this patch:

go test ./pkg/csi/cinder -run TestCreateSnapshotBackupGetBackupByIDError -count=1

Before the fix this panics in CreateSnapshot.
With this patch it returns a normal gRPC internal error, so no crash, nice and boring.

Release note:

NONE

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