removing credentials should first validate they are not in any current use

[mitechie]

If a user were to attempt to remove a credential from their JAAS account that is in use with a current model it should fail with a clear indication that it's used in models X, Y, Z and that the user needs to remove those models first.

At some point in the future (pending https://bugs.launchpad.net/juju/+bug/1713582) it would be good to focus the user on moving to different credentials vs removing the models as a non-destructive path forward.

[frankban]

I think this is already fixed (or in the process to be fixed) in Juju. We could surely improve the UX by not allowing credentials to be removed if still used in models.