Reproducer:  
From external logins tab under user, try to attach a GH account that is already associated with another user. The OAuth callback fails with an error as expected, but when on new UI, this error redirects to old ui page. This should not happen and instead needs to display error in React component. 
Note: There is already an OAuthError.tsx component defined for this, but this might need a change in the backend API for how it should be handled for react since for Angular the api returns a complete HTML error page