Display a meaningful error when no tenant is configured

[jibel]

Test Case

1. Remove the client_id and the issuer from broker.conf so you just have the sections

[oidc]

[users]

2. Login with a remote user
3. Select a broker other than local

Expected result

An error is displayed that a user can understand

Actual result

root@ls:~# login [email protected]
  Select your provider

  1. local
> 2. Microsoft Entra ID

can't select broker: The name com.ubuntu.authd.MSEntraID was not provided by any .service files

Login incorrect
ls login:

[denisonbarbosa]

This is not something that we can show on authd. If you don't have these values configured, the broker service won't even start, so you should take a look at the service logs for the broker instead.

[adombeck]

Similar issue: #466