Error responses v2

Error shape

{
  "error": {
    "code": "connector_unauthorized",
    "message": "The connector's credentials were rejected by the source system.",
    "request_id": "req_c93a10"
  }
}

Breaking change from v1

v1 returned bare error strings. v2 always wraps errors in an error object with a stable code field — match on code, not on message text.