Return the original file when instrumentation failed

We've recently hit a case where some of our old JS (something I'm trying to rewrite) failed to instrument. As a result, everything depending on it failed.

Ideally if a file fails to instrument, it should still be served in it's original form and the warning logged.

I couldn't see any existing way to actually log this. It doesn't make sense to log this to the browser because we want the developer to know (and they aren't looking at a browser). For the moment I've used console.log, but happy to take advice on a better way of doing this within Yeti.