namespace: Provide a special status cause when a namespace is terminating
namespace: Provide a special status cause when a namespace is terminating

Clients should be able to identify when a namespace is being terminated and
take special action such as backing off or giving up. Add a helper for
getting the cause of an error and then add a special cause to the forbidden
error that namespace lifecycle admission returns. We can't change the forbidden
reason without potentially breaking older clients and so cause is the
appropriate tool.

Add `StatusCause` and `HasStatusCause` to the errors package to make checking
for causes simpler. Add `NamespaceTerminatingCause` to the v1 API as a constant.

Kubernetes-commit: a62c5b282fda7c0832d329cde45e5e0a836924e8

kubernetes/api