Comment 5 for bug 1619193

We should catch both cases, not signed the developer agreement and missing a short namespace (to later be replaced with an SSO username check), on snapcraft login.

In the case of registration failed:
```
$ snapcraft login
Email:
Password:
Two-factor:

You need to sign the developer agreement to push snaps. Please visit http://… and log in again.
```

https://docs.google.com/document/u/1/d/1FCPYVcb01XqRoYXCTardWqF7y04Az9JkDPDu7YnvtTU/edit#heading=h.cx2d9tfvjapo

If the short namespace is missing:
```
$ snapcraft login
Email: <email address hidden>
Password:
Two-factor:

You need to set a username. It will appear in the developer field alongside the other details for your snap. Please visit http://… and log in again.
```

https://docs.google.com/document/d/1FCPYVcb01XqRoYXCTardWqF7y04Az9JkDPDu7YnvtTU/edit#heading=h.6xbdc5oax4o2