fixed error was not forwarded

[AndyOGo]

fixes #35

As described in https://github.com/sass/node-sass#importer--v200---experimental

importers can return error and LibSass will emit that error in response. For instance:

done(new Error('doesn\'t exist!'));
// or return synchornously
return new Error('nothing to do here');

[AndyOGo]

@Snugug @xzyfer
Hey guys.
Does any one of you two has some time check the outstanding PRs?
Or is there no plan to maintain this repo.
To be honest I find it really useful and want to contribute, but if no admin is maintaining this, than it will die...

[AndyOGo]

@metaskills @JohnAlbin @misscs @codingdesigner @iamcarrico @scottkellum
Does anyone of you guys have admin access to this repo?
I would really like to get some bug fixes out, but it doesn't seem to be maintained anymore.
I really find it very useful, though some bugs are addressed with open PR's, they don't get merged...

[AndyOGo]

if anyone care we forked this repo and republished it on npm as @axa-ch/node-sass-import-once
https://github.com/axa-ch/node-sass-import-once

[AndyOGo]

@Snugug
This PR is almost a year old...

[Munter]

I'm hitting this problem again now and am blocked on it. I'd prefer not to have to create fork

[AndyOGo]

@Munter
I forked it ages ago...
https://github.com/axa-ch/node-sass-import-once

[toshi-toma]

I also face this problem. Please merge this PR.