Show an error if user cannot login with WebAuthn
Show an error if user cannot login with WebAuthn

This follows the same approach as for registration [1]. None of
this code is tested, as:

- We don't have a way to test inline JS.
- The risk of this code not working is low.
- We might change the approach in future [2].

[1]: https://github.com/alphagov/notifications-admin/pull/3886
[2]: https://github.com/alphagov/notifications-admin/pull/3886#issuecomment-841128380

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin