Error when using darwin goon in linux docker container could be clearer

If you're using docker on OSX to run your Elixir project and you download the darwin version of goon, thinking you need the OSX version, you'll get

```
14:51:56.559 [info] Application porcelain exited: Porcelain.App.start(:normal, []) returned an error: "goon executable at /code/goon does not support protocol version 2.0"
```

But the actual problem is you needed to install the linux amd64 version of goon. I don't know if it would be possible to detect you're using the wrong OS's goon, but it would be helpful.