Template error raised in an {% extends %} child template shows incorrect source location on debug page

[Matt Westcott]

If an error occurs in the processing of a template tag on a template that extends another via {% extends %}, the "Error during template rendering" section of the debug page incorrectly shows an excerpt from the parent template rather than the child.

To reproduce:

base.html:

{% block body %}{% endblock %}

home.html:

{% extends "base.html" %}

{% block body %}
    {% include "does_not_exist" %}
{% endblock %}

views.py:

from django.shortcuts import render

def home(request):
    return render(request, 'home.html')

The resulting error page displays:

Error during template rendering
In template /Users/matthew/Development/tbx/wagtail/debug/brokendebug/core/templates/base.html, error at line 0

does_not_exist
1	{% block body %}{% endblock %}
2	

instead of the expected error output:

Error during template rendering
In template /Users/matthew/Development/tbx/wagtail/debug/brokendebug/core/templates/home.html, error at line 4

does_not_exist
1	{% extends "base.html" %}
2	
3	{% block body %}
4	    {% include "does_not_exist" %}
5	{% endblock %}
6	

Bisecting shows that this bug was introduced in #27956 (e643ba8bcf0b1da517cbab689ac157ee031202a3), which was a fix for the opposite case: an error occurring on the parent template. (In the above test case, moving the line {% include "does_not_exist" %} to base.html turns it into a test case for #27956, and fails on pre-e643ba8bcf0b1da517cbab689ac157ee031202a3 revisions as expected.)