isaacs/npm — pull request #2560 by heycalmdown (2012-06-24 09:44:54+00:00)
Title: Avoid install failure from that the `treeify` get null

As you can see at [here](https://github.com/isaacs/npm/blob/master/lib/install.js#L629) a failure from an optional dependency eats an error. It cause a problem by that both of `err` and `installedWhat` could be null at same time if the optional package was an only package which needed to be installed.