Small fixes and cleanups

[gchazot]

Purpose of PR

• Small cleanups as seen during my work on #147
• Fix issues with k8s namespace handling:
  • the "default" namespace cannot be deleted, that's forbidden by k8s
  • don't try to delete the target namespace if it doesn't even exist
• Fix a bug with gathering annotations when some objects in the namespace may not have any. This happens only if the kubetools application is deployed to a namespace shared with resources deployed with other tools, and that may not have any annotations, or even be of a Kind that doesn't allow annotations.

[gchazot]

Thanks for the reviews @DilaraOflaz @tmwoodruff