handleError method
Implementation
void handleError(Exception exception) {
  if (exception is! FirebaseAuthException) {
    value = AuthFailed(exception);
    return;
  }
  if (exception.code == 'account-exists-with-different-credential') {
    final email = exception.email;
    if (email == null) {
      value = AuthFailed(exception);
      return;
    }
    findProvidersForEmail(email, credential: exception.credential);
    return;
  }
  value = AuthFailed(exception);
}