k6 will crash while trying to kill a "forbidden" namespace

[dgzlopes]

For example, while trying to kill kube-public.

Trace:

ERRO[0001] GoError: namespaces "kube-public" is forbidden: this namespace may not be deleted
running at reflect.methodValueCall (native)
default at DeleteRandomNamespace (file:///home/dgzlopes/go/src/github.com/grafana/k6-jslib-chaos/src/kubernetes.js:33:33(26))
        at file:///home/dgzlopes/go/src/github.com/grafana/k6-jslib-chaos/example.js:13:40(25)
        at native  executor=per-vu-iterations scenario=default source=stacktrace

We should handle this gracefully.