login: In realm-input, give "subdomain required" feedback, when servers should but don't

[chrisbobbe]

In particular, this TODO on our getServerSettings request:

// TODO(#107): Perhaps detect realmless root domain, for more specific onboarding feedback.
//   See thread, and the zulip-mobile code and chat thread it links to:
//     https://github.com/zulip/zulip-flutter/pull/55#discussion_r1160267577

We could have code in our getServerSettings binding along the lines of that binding's counterpart in zulip-mobile:

  if (realm_name == null) {
    // […]
    //
    // This error copies the proper error that servers *sometimes* give when
    // the root domain is requested and there's no realm there. [1]
    // Specifically, servers give this when
    // `ROOT_DOMAIN_LANDING_PAGE = True`. That circumstance makes no
    // difference to mobile.
    //
    // [1] Observed empirically. For details, see
    //   https://chat.zulip.org/#narrow/stream/412-api-documentation/topic/.2Fserver_settings.3A.20.60realm_name.60.2C.20etc.2E/near/1332900 .
    throw new ApiError(400, { code: 'BAD_REQUEST', msg: 'Subdomain required', result: 'error' });
  }

[chrisbobbe]

We could have code in our getServerSettings binding along the lines of that binding's counterpart in zulip-mobile:

If taking this approach, we should probably wait for #37, so we have a counterpart for ApiError to throw.