templater: add hint to template parse errors to help locate issues

Authored by ryanmce.

Description

templater: add hint to template parse errors to help locate issues

Previously, we would print the error name and location, but this isn't as
helpful as we can be. Let's add a hint that shows the location where we
encountered the parse error.

Differential Revision: https://phab.mercurial-scm.org/D2608