Can't find main.debug.css error

When setting up a new ckan instance and running it on debug mode, this is a common error. The fix is in the [Frontend Guidelines](http://docs.ckan.org/en/latest/contributing/frontend/index.html). However, that is now out-of-date because npm is hipster

![hipster cat](http://stream1.gifsoup.com/view5/3553806/dubstep-hipster-cat-o.gif)

By which,I mean, the version of npm in Ubuntu repositories are too old.

1) We should mention this under Troubleshooting.
2) Update the frontend guidelines to talk about installing nodejs from [PPA](https://launchpad.net/~chris-lea/+archive/node.js/)