[Question] Can you access preprocessor errors programatically?

I'm running karma programatically through the `Server` API and was wondering if there was a good way catch preprocessor errors when they occur.

Right now, if a preprocessor (babel for example) fails, I'll just see a message logged from Karma, but the server will still start up.

I'd like to be able to cancel the whole process, or at least catch the error when it occurs. I tried tracing my way through the source, and couldn't exactly understand what happens when a preprocessor invokes it's callback with an error: https://github.com/karma-runner/karma/blob/master/lib/preprocessor.js#L26

Any advice?