Fix "Unknown auth error" when account is actually blocked

[kaxap]

Some time ago a weird exception started to appear: "Unknown API auth error".
After debugging I noticed that an app token is never acquired and

response = self.http.get(
            'https://oauth.vk.com/authorize',
            params={
                'client_id': self.app_id,
                'scope': self.scope,
                'response_type': 'token'
            }
        )

is actually returning "Your account has been banned" page. So I have added two lines to clarify the auth exception.