Mock every HTTP error

Most error-handling code is written from imagination and never executed until a real outage runs it for the first time. This mock gives every failure its own address: call /404 to get a 404, /500 to get a 500, /429 to get a rate limit with a Retry-After header. Point your client at each in turn and find out what it actually does.

{
  "error": "bad_request",
  "message": "The request could not be understood."
}