Add an error message for missing dependencies.

This ensures that something is printed out for users who didn't add a mapping for one of their dependencies. Either they missed a deps.js file or they didn't preprocess all of their source files. The current behavior adds an undefined filepath which ends up causing the karma web-server to blow up when it trys to do indexOf on undefined.

I'm sure it would be better to actually get the real karma logger here, but I didn't see any examples of doing that in any of the other plugins. So for now I'm just using a console.error.