Prevent SystemJS from swallowing helpful errors

*Should be merged after #6 as it was branched off of that*

## Problem
I was not attaching an error callback to the `System.import` promises that load the test specs. Turns out this causes SystemJS to silently fail when it came across a require statement that it didn't like. This would cause the promise never to resolve and as a result the tests would never run (`karma.start()` only gets called when all the promises resolve) - making debugging very difficult.

## Solution
Add an error handler that throws any errors SystemJS has.