ViewResponse is missing error metadata

[stevekuznetsov]

In the Slack playground if I send an invalid modal view request I get this as a result:

{
    "ok": false,
    "error": "invalid_arguments",
    "response_metadata": {
        "messages": [
            "[ERROR] invalid `trigger_id` [json-pointer:\/trigger_id]"
        ]
    }
}

This is unmarshalled into:

{
  "ok": false,
  "error": "invalid_arguments",
  "view": {
    "ok": false,
    "error": "",
    "id": "",
    "team_id": "",
    "type": "",
    "title": null,
    "close": null,
    "submit": null,
    "blocks": null,
    "private_metadata": "",
    "callback_id": "",
    "state": null,
    "hash": "",
    "clear_on_close": false,
    "notify_on_close": false,
    "root_view_id": "",
    "previous_view_id": "",
    "app_id": "",
    "external_id": "",
    "bot_id": ""
  }
}

Which is missing the response metadata fields, making the error hard to debug.