login: Interpret API errors for the user

[chrisbobbe]

Needs #37.

When the user enters a realm URL, then their username/password, we should give more specific feedback on failure than "Could not connect" and "Login failed", respectively.

We've written TODO comments for that in the code:

• This TODO in error handling on our getServerSettings request:

          // TODO(#105) give more helpful feedback; see `fetchServerSettings`
          //   in zulip-mobile's src/message/fetchActions.js. Needs #37.
          showErrorDialog(context: context,
            title: 'Could not connect', message: 'Failed to connect to server:\n$url');

• This TODO in our error handling on fetchApiKey:

          // TODO(#105) give more helpful feedback. Needs #37. The RN app is
          //   unhelpful here; we should at least recognize invalid auth errors, and
          //   errors for deactivated user or realm (see zulip-mobile#4571).
          showErrorDialog(context: context, title: 'Login failed');

This issue is for tracking those TODOs.

[gnprice]

For cross-reference: the zulip-mobile issue linked in one of those comments is:

• Handle new USER_DEACTIVATED and REALM_DEACTIVATED API error codes. zulip-mobile#4571