RPC API should return error message when failing to parse JSON

[alecalve]

Currently, when sending a RPC request with unescaped chars like in #387, lightningd returns an empty string when it should instead return a valid JSON RPC response like:

{
  "id": null,
  "error": {
    "code": -32700,
    "message": "Parse error"
  }
}

[cdecker]

Should be addressed by #957

[bitonic-cjp]

This should be open until #1118 (or another fix) is merged.