Use Jason.encode! when sending errors

[daskycodes]

At the moment Twirp.Plug.send_error/2 uses Twirp.Encoder.encode/3 for encoding the error.

This leads to a problem where the defimpl Jason.Encoder for the Twirp.Error struct is not being used, and thus the :__exception__ field is being sent with every error response.

The twirp specification v7 describes how error responses must be encoded:

 {
  "code": "permission_denied",
  "msg": "Thou shall not pass",
  "meta": {
    "target": "Balrog",
    "power": "999"
  }
}

code and msg are required while meta is optional.

The twirp-elixir implementation will send the following:

{
  "__exception__": true,
  "code": "permission_denied",
  "msg": "Thou shall not pass",
  "meta": {
    "target": "Balrog",
    "power": "999"
  }
}

The twirp go implementation is very strict about the errors

var tj twerrJSON
dec := json.NewDecoder(bytes.NewReader(respBodyBytes))
dec.DisallowUnknownFields()
if err := dec.Decode(&tj); err != nil || tj.Code == "" {
	// Invalid JSON response; it must be an error from an intermediary.
	msg := fmt.Sprintf("Error from intermediary with HTTP status code %d %q", statusCode, statusText)
	return twirpErrorFromIntermediary(statusCode, msg, string(respBodyBytes))
}

This leads the Go implementation to handle the error as if it originated from an intermediary (e.g. a reverse proxy) and translating the HTTP status code 404 as a bad_route instead of the original error.

This PR makes use of Jason.encode!/1 in the Twirp.Plug.send_error/2 to fix this issue, as it will use the protocol implementation found in Twirp.Error.