Remove Kubernetes exception handling from abstract auth handler

[andrewazores]

Looks good. I'd like to come back around and refactor this at some point so that there are no Kubernetes details leaked into this abstract handler, but that's a task for later.

Originally posted by @andrewazores in https://github.com/cryostatio/cryostat/pull/692#pullrequestreview-765981065

The abstract auth handler - or any other platform agnostic components - should have 0 knowledge of Kubernetes exception types, or any other platform-specific details.