Display a meaningful error when no tenant is configured

## 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 user@domain.tld
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:
```