HTTP status codes
Also written: 400, 401, 403, 404, 408, 413, 422, 429, 500, 502, 503, 504, 529, 5xx, status
Numbers that say how a request went: 400, 404, 413, 422 your request is wrong; 401/403 bad or revoked key; 408 timeout; 429 too many requests (rate limit) or out of quota; 500, 502, 503, 504 server or provider trouble (503 is also the service's own "busy"); 529 provider overloaded; 0 means no answer came back.