Improve error messages on names registration and usage to separate some cases

[Facundo Batista]

When trying to use a charm that the user doesn't have permissions, the result is "Name NAME not found in the charm namespace" (code 'resource-not-found').

This gives the user the impression that NAME is not registered yet, but then when trying to register it it receives "Name NAME is already registered" (code 'already-registered'). And this may not even be the case for reserved names.

We think we can improve the user experience in two ways:

1. provide errors distinguishing the case of "the charm does not exists" than "the charm exists but you don't have credentials to see it"

2. provide errors distinguishing the case of "the charm name is already registered" than "the charm name is reserved"

This idea was originated in this Charmcraft request:

https://github.com/canonical/charmcraft/issues/337

Thank you!!

[William Grant (wgrant)]

It's definitely unacceptable from a security perspective to provide an oracle for whether a charm exists that you're not allowed to see. We could potentially justify distinguishing reserved and already registered, but it would need some thinking around security.