Adds proper emitting of errors.

As far as I can understand there are no proper way of handling errors in this module right now. All modules are suppressed and simply logged out to the console (causing the error #82 - which is a bug with this plugin). There where proper handling previously, but after changing to gulp-intermediate, this module is no longer emitting it's error. 

This adds errors the "proper way" by emitting them to the stream – allowing for errors to be caught.

_As a side note: The test is failing (and was previously due to a separate issue), and adding a listener for errors show what fails._
Preserve code style here, just returning intermediate is fine.