fix: Fixes Unauthorized error when listing K8s nodes

[Gmerold]

Description

When kube-apiserver isn't ready, fetching list of K8s nodes fails with Unauthorized error. This fix handles such situation by catching the error and logging the relevant message rather than raising an error which puts the charm in the error state.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library