When a Liquid Exception is caught, show the full path (rel to site source)

Instead of just the basename of the page or post that has encountered a Liquid
exception, it will print the path of the item being processed, relative to the
site source.

E.g.


- `docs/index.markdown` has a problem
- outputs `Liquid Exception: ... index.markdown`


- `docs/index.markdown` has a problem
- outputs `Liquid Exception: ... docs/index.markdown`