Generate structured errors

Generate consistent, structured response bodies on errors. Include a machine-readable error id, a human-readable error message, and optionally a url pointing the client to further information about the error and how to resolve it, e.g.:

HTTP/1.1 429 Too Many Requests

{
  "id":      "rate_limit",
  "message": "Account reached its API rate limit.",
  "url":     "https://docs.service.com/rate-limits"
}

Document your error format and the possible error ids that clients may encounter.