The code introduced by #18185 gets the error from response after it was processed by goth.
That is incorrect, as goth (and golang.org/x/oauth) doesn't really care about the error, and it sends a token request with an empty authorization code to the server anyway, which always results in a `oauth2: cannot fetch token: 400 Bad Request` error from goth.
It means that unless the "state" parameter is omitted from the error response (which is required to be present, according to [RFC 6749, Section 4.1.2.1](https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.2.1)) or the page is reloaded (makes the session invalid), a 500 Internal Server Error page will be displayed.
This fixes it by handling the error before the request is passed to goth.
1e2c2edab6