The error message was unnecessary cryptic. `snapshot-[name]` notation
was only used here and hard to understand.
Instead it should say `snapshots on "..." snapshotter`.
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
Deleting a non-empty namespace fails with
> namespace must be empty: failed precondition
This change improves the error message by listing the types of
the objects in the namespace that prevent deletion.
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
