I don't quite like the idea of returning the logs with the same request, maybe as an option. I don't see a problem with keeping the logs of the js processing next to the logs of splash. In any case I want to keep this PR as simple as possible.

We are going to be doing more JS related changes soon (I have another PR ready to work on top of this) and we can revisit this matter later. One option is to keep the render.js endpoint simple and then when we integrate the js render with the render.json we can add the logs outputting option there.