Group snapshot not taken on conflict v2

[jsafrane]

This is continuation of #1280
The patch did not help, now the e2e test fail just a bit later.

Test results from a downstream e2e job:

• overview
• snapshot-controller logs

The test panicked here:

k8s.io/kubernetes/test/e2e/storage/testsuites.(*VolumeGroupSnapshottableTestSuite).DefineTests.func1.1.5({0x7efc10eca568, 0xc0032afa40})
	k8s.io/kubernetes/test/e2e/storage/testsuites/volume_group_snapshottable.go:196 +0x1054

fail [runtime/iface.go:275]: Test Panicked: interface conversion: interface {} is nil, not string
fail [runtime/iface.go:275]: Test Panicked

I.e. the snapshot controller declared a GroupSnapshot readyToUse=True, but it did not file status.volumeSnapshotInfoList[]. Which is a bug, this must never happen.

The real root cause of #1280 seems to be deeper than simple error checks.