An example of a good error message coming from an HTTP API (illustrative for http://milancermak.posterous.com/http-apis-and-errors-doing-it-the-right-way)

gistfile1.http

HTTP/1.1 402 Payment Required
Content-Type: application/json

{
  "title": "End of trial",
  "description": "Unfortunately, your trial period has ended. To continue using the application, you need to purchase a subscription.",
  "explanation_url": "https://example.com/troubleshooting/payment-required",
  "trace_url": "https://apilog.example.com/f26a3e1dc"
}