Raise any JS error when prerendering

A simple attempt at solving #42. Even with better_errors it's not pretty:

![image](https://cloud.githubusercontent.com/assets/2231765/4274518/a48b0068-3cf4-11e4-8ba4-78270308e522.png)


But I'm not sure what else can be done.  Have a look at how `ProgramError` is raised: https://github.com/sstephenson/execjs/search?utf8=%E2%9C%93&q=ProgramError

It looks like it only comes with a `message` -- no stack trace. Any ideas on how to make this more useful?