Structured error replies

To allow better messages and logs, KACLS operation failures should return a structured JSON reply in the body. The standard HTTP status codes should be used.

JSON representation 
{
  "code": int,
  "message": string,
  "details": string
}
Fields
code

int

Standard HTTP status code
message

string (UTF-8)

A human readable error message.
details

string (UTF-8)

More details about the error, such as a stack trace.