storage: etcd: TestWatchError: improve readability
storage: etcd: TestWatchError: improve readability

This test, as written, is *extremely* cryptic and hard to parse. Add a
comment and stop intentionally ignoring an error that only needs to be
ignored if we're being cryptic.

Signed-off-by: Steve Kuznetsov <skuznets@redhat.com>

Kubernetes-commit: 50eed81923495f5ee1ac44436676ddbaf2a380fe

kubernetes/apiserver,kubernetes/apiserver