Clearer sign-in message when an end user's setup isn't complete

An end user who opened their welcome email but didn't complete mobile verification saw only "An error has occurred" when trying to sign in — no indication that registration was unfinished, and nothing to act on. Most ended up contacting the help desk rather than simply finishing their setup.

Sign-in attempts in that situation now return a clear message:

> We couldn't sign you in. Your account setup may not be complete. Please check the welcome email we sent you, or contact your administrator for help.

The same wording appears in both the Self-Serve web app and the Self-Serve mobile app.

The message is deliberately the same for every sign-in failure — it's returned whether or not the email matches a known account, and it doesn't say which setup step is outstanding. That keeps the sign-in screen from revealing which addresses are registered with your organisation, or how far a given user has progressed.