Code review comment for ~silverdrake11/landscape-charm:install-and-bootstrap-bug_lndeng-466

I learned that Juju's exponential retry behavior can be triggered by raising an exception. Some things are useful for this such as this install bug/ race condition. The install initially fails but will resolve itself when it's ready. Other things are not useful for this, like erroring out on a mistake in the config (retrying doesn't change the config value). So because of this, raising an exception can be useful for other charm functions in the future or when needed.