selinux::login: Miscellaneous fixes

[EmRowlands]

Pull Request (PR) description

This PR provides a small number of fixes for the selinux::login defined resource, including:

• A reference to Nil in the provider.prefetch method
• Changing the resource's arguments correctly shows up as changing the mapping, not creating a new one and deleting the previous version
• Additional error handling in the helper script

This Pull Request (PR) fixes the following issues

n/a

[EmRowlands]

I'll mark this as WIP until I've fixed the CI issues

[EmRowlands]

This is now fully rebased onto master, and is ready for re-review.