Errors

Errors use the standard envelope with a non-SUCCESS code:

{
  "code": "VALIDATION_ERROR",
  "message": "phone: cannot be blank."
}

Branch on the HTTP status and code, not on message.

HTTP	Codes
400	`VALIDATION_ERROR`, `INVALID_JID`, `ALREADY_LOGGED_IN`, `DEVICE_ID_REQUIRED`
401	`AUTHENTICATION_ERROR` (device not logged in); also Basic Auth failures (no JSON `code`)
404	`NOT_FOUND`, `DEVICE_NOT_FOUND`
408	`CONTEXT_ERROR`
429	`WA_REACHOUT_TIMELOCK`
500	`INTERNAL_SERVER_ERROR`, `QR_CHANNEL_ERROR`, `SESSION_SAVED_ERROR`, `UPLOAD_MEDIA_ERROR`, `WEBHOOK_ERROR`, `INVALID_WA_CLI`
503	`DEVICE_MANAGER_UNAVAILABLE`
504	`GATEWAY_TIMEOUT`

Meanings, fixes and retry guidance: Error handling.