Actually extend from Error

[seangenabe]

It's not simple trying to extend from Error. I have observed that errors from this module lack prototype and stack (which is very important for debugging). I've done some research and from jashkenas/coffeescript#2956 it seems that Coffeescript does not intend to fix that, though there's a suggestion there, maybe we can try that?

class MyError extends Error then constructor: -> super

[jeremyfa]

Sounds reasonable if it fixes the issue, feel free to make a PR to develop with this change