Better error message for add-model with no credential

[Vitaly Antonenko]

Let's add a new user with superuser controller access and login to controller:

```
juju bootstrap lxd lxd
juju change-user-password
juju add-user superadmin
juju grant superadmin superuser
juju change-user-password superadmin
juju logout
juju login
```
After login as `superuser`, let's check our rights:

```
juju users
```

Add try to add model

```
juju add-model test-model
```

The result is the ERROR:

```
ERROR getting credential: cloud credential "lxd/superadmin/lxd" not found
```

[Ian Booth (wallyworld)]

This is expected - just because you have been given permission to add a model to a cloud, you can't do it without supplying your own credential. You don't get to use someone else's credit card to pay for your cloud use :-)

You need to use the --credential argument to add-model.

The error is telling you that it cannot find your credential for that cloud. Adding an extra line of text to better explain the situation would be helpful.

[Evanson Mwangi (mr-parish)]

Fixed here https://github.com/juju/juju/pull/14582