Error envelope and common codes.

Errors

Envelope

{
  "error": {
    "code": "not_found",
    "message": "Market not found: btc-updown-99m",
    "request_id": "req_7c2a91"
  }
}

Code	HTTP	Meaning
`unauthorized`	401	Missing or invalid key
`forbidden`	403	Plan does not include this source
`not_found`	404	Unknown market or file
`validation_error`	400	Bad params
`rate_limited`	429	Over quota
`internal`	500	Our fault — retry with backoff

Include request_id when contacting support.