Correct message when registrations is blocked

Problem/Motivation

Currently when the registration is blocked due to some error this is shown,

'Only existing users can log in. Contact system administrator.'

    // Check if site configuration allows new users to register.
    if ($this->registrationBlocked()) {
      $this->loggerFactory
        ->get($this->getPluginId())
        ->warning('Failed to create user. User registration is disabled in Drupal account settings. Name: @name, email: @email.', array('@name' => $name, '@email' => $email));

      drupal_set_message($this->t('Only existing users can log in. Contact system administrator.'), 'error');

      return FALSE;
    }

Proposed resolution

Change the message to correct one.

Remaining tasks

• Define which message will be displayed.
• Create patch