Stream behavior with errors

If the call to the `download` function below fails with an error, `stopOnError` will cause the `Promise` to `reject`. Since this is doing `parallel(10)`, will this code wait for the other 9 `download` operations to finish, or will it terminate them right away when it encounters the first error?

How will the behavior be different if I use `mergeWithLimit(10)` instead of `parallel(10)`?

```js
await new Promise((res, rej) => highland(range)
.map(page => highland(download(page)))
.parallel(10)
.stopOnError(rej)
.done(res));
```

Start with the implementations or API documentation for parallel, stopOnError, and mergeWithLimit, then exercise the shown download example with one failing operation. Compare whether other active downloads complete or are cancelled, and document the observable difference between the two stream operators.

Written by the indexing model from the issue text.