Process hangs in event of transpilation error

[waynegerard]

If there's a transpilation error, we get a nice log message indicating where the error occurred, but the process hangs indefinitely because the finish callback is never executed (rightfully so, since it would probably cause many other errors down the line).

Unfortunately this has the nasty side effect that anything depending on the process to finish (e.g. a deployment script) will hang indefinitely as well. I think it'd be nice to add the ability to have a callback executed in the event of a transpilation error. I've made the corresponding change here:

https://github.com/waynegerard/karma-coffee-preprocessor/commit/2255cb4687debf080eebdb9144ad02665e9726a0

But just wanted to hear feedback before making a pull request.