Auth UI and OAuth: show auth errors on screen

[Martinsos]

Note

This will be closed by #1851

We had the case (in wasp 0.12) where:

1. User has email field with unique constraint. It is a field we set with email via userSignupFields on successful signup.
2. I sign up with Google, with email address E.
3. I try to sign up with Github, with same email address E, but it just returns me to login.
   I get following errors in the console:
   

Instead, I would expect that some kind of nice error is shown on the screen. I am guessing error gets thrown in userSignupFields -> well, actually as a result of executing the setters/gettters defined there -> but it doesn't get displayed by Auth UI for some reason.

[infomiho]

Done in #1851