Add message about cleaning cache upon error

Full credits goes to http://blogs.msdn.com/b/matt-harrington/archive/2012/02/23/how-to-fix-node-js-npm-permission-problems.aspx

There is plenty issues floating around in repos of other packages. Unfortunately it seems that not many people are reading [Troubleshooting](https://github.com/npm/npm/wiki/Troubleshooting) which mentions it. I wasn't looking there too honestly.