improve error messaging if required config options are not defined

For both serving and eventing, if the user does not specify the namespace they will get a nondescript lightkube error that makes it difficult to understand a config option has been missed. Improve this error message, or possibly validate the config values before starting the charm.

Reproduce the missing-namespace case for both serving and eventing, then inspect where charm configuration is handled before startup. Done means users receive a clear message identifying the missing namespace instead of a nondescript lightkube error, or the configuration is validated before startup.

Written by the indexing model from the issue text.