allow custom error code

[hannaeko]

Allow setting custom error code (#7).

Throwing an error like this one:

die { code => -32000, data => { some => "data" }, message => "Internal error 012" };

results on the following json message:

{
  "id": 1,
  "error": {
    "data": {
      "some": "data"
    },
    "code": -32000,
    "message": "Internal error 012"
  },
  "jsonrpc": "2.0"
}