Output yaml path

Also, yamltester gives the line number for the start of the document.

I only did RulesTest and YamlTester.
I can't think of anything, but the build failed because filename was null (handled) and I figured making this a bit more robust couldn't hurt. But you're right, as long as filename isn't null, nothing should be thrown, so I'll just remove it, and keep the code cleaner.