Add error message for if the users endpoint is blocked #20

Currently the extension kind of breaks if you try to log in with the users endpoints blocked. The avatar doesn't show up and the Log In button is still displayed. The fix for this should likely be two-fold:

1. Show a warning when the user puts in their Site URL that the users endpoint is blocked and might not make the avatars display properly.

2. Handle the case where the JWT is gotten properly but the extension can't fetch the avatar, maybe just display the user's login rather than their display name/avatar.