Capacity drill-down pages show a generic error instead of 403-specific copy

[lexfrei]

ClusterUsageResourcePage and StorageClassUsagePage render a generic "Failed to load…" message on any list error. ClusterStorageSection, in the same Capacity area, distinguishes a 403 ("You do not have permission…") from other failures. For consistency the two drill-down pages should do the same.

Both pages sit behind CapacityAdminGuard (gated on nodes/list), so the realistic 403 case is a user who can list nodes but not the pods/PVCs the drill-down reads.

Suggested fix: reuse the K8sApiError status check from ClusterStorageSection, with a test for the 403 branch.