Display more clear error when OIDC group is not synchronized

For example, authenticating via Google account, when group is not specified in this field

mobile client will show `400` and unclear error:
```
error] | 10:41:30 91ms | OpenID MFA polling error: DioException [bad response]: This exception was thrown because the response has a status code of 400 and RequestOptions.validateStatus was configured to throw for this status code.
The status code of 400 has the following meaning: "Client error - the request contains bad syntax or cannot be fulfilled"
Read more about status codes at https://developer.mozilla.org/en-US/docs/Web/HTTP/Status
In order to resolve this exception you typically have either to verify and fix your request code or you have to fix the server code.
```

We should add more clear errors, (like we do in core).

Start in the mobile client's OpenID MFA polling and error-handling flow, then compare its behavior with the clearer errors in core. Reproduce authentication with an unsynchronized OIDC group and trace the 400 response. Done means the client presents a specific, understandable group-synchronization error instead of the generic Dio message.

Written by the indexing model from the issue text.