[jvm] better error checking when we can't find test file

There are two different ways that finding the test file can fail.

1. you're using a `Selfie.vcrTestLocator`, and the VCR was created somewhere far from the test itself, its possible that comments in the test file won't get picked up at all
2. if you put a test into `selfie.` or `com.diffplug.selfie.`, it's possible that it will be ignored when determining where the test file is

These can be fixed by a more intensive search, they can also be fixed with better error messages.