Fix error file reporting

This PR fixes 2 quirks in jekyll's liquid error reporting, specifically which file the error occured on:

* Firstly, it extends #1596 to work for parse-time errors (such as unknown tags), while it previously only worked for render-time errors.
* Secondly, the original report #1591 related to layouts and was thus not fixed by #1596. This PR now also fixes the error reporting for liquid errors in layouts.

Sorry for not testing this before opening #1596, but I had no ruby install available. It's tested this time, for both cases.

//cc @IQAndreas, @mattr-