unfriendly build.rs errors for stylus

[budziq]

Hi,

As mentioned on #292
the build.rs output when of node / npm / nib / stylus is missing are pretty cryptic.

Would you be interested in a PR to make it more friendly?

Moreover, I guess that pulling error-chain as a dependency is out of the question?

[azerupi]

Would you be interested in a PR to make it more friendly?

Of course! I will never say no to a PR that improves error messages. 👍

Moreover, I guess that pulling error-chain as a dependency is out of the question?

It is not, I have actually considered using it for some time but never had the time to take a closer look at it. I've heard many people praise its usefulness and simplicity so I am willing to try it out. If you want to pull it in as a dependency I am fine with it.