juju-upgrader should warn you when you're not in the controller model

[Nick Moffitt]

I had some rather perplexing behaviour when doing a juju-upgrader run, simply because I'd juju switched to another model and forgot to switch back. juju-upgrader should notice when you're not in the controller's own model, and advise you to change (and possibly offer to try a simple `juju switch controller` first?)

[Paul Collins (pjdc)]

I just ran into what I assume is the same problem:

jenkins@juju-59050e-is-mojo-ci-0:~/juju-upgrader$ ./juju-upgrader --controller --models --percent 100 --version 2.2.9
[...]
2018-01-10 20:25:56 +0000 Create backup
ERROR backups are not supported for hosted models
jenkins@juju-59050e-is-mojo-ci-0:~/juju-upgrader$ _

"juju switch controller" and it worked.