What happens when VIES is temporarily unavailable?

VIES is shared infrastructure across 27 national tax authorities and has occasional outages, sometimes affecting just one country. When a check can't be completed and no fallback succeeds, vatnode returns HTTP 503 with the code VIES_UNAVAILABLE — which is very different from an "invalid" result. The right pattern is to treat it as a signal to retry later rather than to reject the customer; for several countries vatnode's national-registry fallback returns a result even while VIES is down.