Updates error message when referencing the wrong path to a BUILD file.

Before, it would just spit out the incorrect BUILD file path.:

  Exception message: BUILD file does not exist at: /Users/zundel/Src/Pants/src/java/com/pants/examples/annotation/exampleTYPO

Now it will read as follows:

  Exception message: BUILD file does not exist at: /Users/zundel/Src/Pants/src/java/com/pants/examples/annotation/exampleTYPO
    referenced from src/java/com/pants/examples/annotation/processor:processor
    referenced from src/java/com/pants/examples/annotation/main:main

Testing Done:
Updated unit tests, ci passed at https://travis-ci.org/pantsbuild/pants/jobs/31812266

Bugs closed: 442

Reviewed at https://rbcommons.com/s/twitter/r/841/