`err` and `cause` should be swapped in some places

https://github.com/ssbc/ssb-ahoy/blob/f6afe79eada23c494b171af86639d4a9a2e87f64/index.js#L43

it should be
```js
      if (err) return cb(Error('ssb-ahoy failed to start SSB stack', { cause: err }))
```
to get correct error propagation