Return gardener user error for unmanaged resource on cluster delete

[aniruddha2000]

How to categorize this PR?

/kind enhancement

What this PR does / why we need it:

this PR handles graceful deletion for 409 error code returned by STACKIT IaaS API and converts it to relevent gardener error.

Special notes for your reviewer:

Breaking changes:

[nschad]

Add the error cases like SubnetInUse to dependenciesRegexp. Don't change the the infraflow part. Don't call DetermineError multiple times.

It's already called here:

gardener-extension-provider-stackit/pkg/controller/infrastructure/stackit/actuator_delete.go

Lines 25 to 30 in a48f725

	func (a *actuator) Delete(ctx context.Context, log logr.Logger, infra *extensionsv1alpha1.Infrastructure, cluster *extensionscontroller.Cluster) error {
	return util.DetermineError(
	a.delete(ctx, log, infra, cluster),
	helper.KnownCodes,
	)
	}

[ske-prow]

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[nschad]

Please do it like this

gardener-extension-provider-stackit/pkg/openstack/client/client.go

Lines 216 to 226 in 281fccb

	func IsNotFoundError(err error) bool {
	if err == nil {
	return false
	}
	if gophercloud.ResponseCodeIs(err, http.StatusNotFound) {
	return true
	}
	return false
	}

Just add a util function within the same file.