Add an exception if Omnikassa is unavailable

[GErpeldinger]

Hello,

We use omnikassa in our application, and sometimes the sandbox environment is unavailable, the problem is that the error that is send when the service is unavaible doesn't speak for itself. We hava a Warning: Trying to access array offset on value of type null send from the class AccessToken line 48.

Maybe a fix can be to see if the access token contain an error message before creating the access token from the json in the method retrieveNewToken in the class ApiConnector like this:

    /**
     * @return AccessToken
     */
    private function retrieveNewToken()
    {
        $refreshToken = $this->tokenProvider->getRefreshToken();

        $this->restTemplate->setToken($refreshToken);
        $accessTokenJson = $this->restTemplate->get('gatekeeper/refresh');

        if (str_contains($accessTokenJson, 'Tijdelijk niet beschikbaar')) {
            throw new \HttpException('OmniKassa is temporarily unavailable', 503);
        }

        return AccessToken::fromJson($accessTokenJson);
    }

What do you think?