[multi-user] Confusing error printed when user without sufficient permissions attempts to add a model

[Peter Matulis]

When a user who has neither been granted 'superuser' controller permissions nor 'add-model' model permissions attempts to use `add-model` a confusing error is printed:

$ juju add-model golf
ERROR cloud "aws" not found

Here, the controller is AWS-based.

After either of the above permissions is granted the command succeeds.

[Richard Harding (rharding)]

Thanks, more clear error messaging is definitely useful.

[Nam Nguyen (nammn)]

This seems to have been fixed some releases ago:

```
❯ juju add-model bla
ERROR you do not have add-model access to any clouds on this controller.
Please ask the controller administrator to grant you add-model permission
for a particular cloud to which you want to add a model.
```