use TEMPLATE_STRING_IF_INVALID format string in blocktrans

[Kevin Turner]

ticket:19915 makes it so that TEMPLATE_STRING_IF_INVALID may show when template strings fail inside blocktrans blocks. It doesn't, however, treat that as a format string in the way that ​FilterExpression.resolve does (as is described in ​the documentation).

[chrismedrela]

To reproduce:

• set TEMPLATE_STRING_IF_INVALID = "Invalid(%s)"

• run python manage.py shell and type:

from django.template import Template, Context
t = Template("""{% load i18n %}
    Outside: {{ missing }}
    {% blocktrans %}Inside: {{ missing }}{% endblocktrans %}""")
print(t.render(Context()))
>>>    Outside: Invalid(missing)
>>>    Inside: Invalid(%s)

Missing variables in blocktrans tag are handled here: ​https://github.com/django/django/blob/326539f6a4936cda031b4a5f7caad788569f3329/django/templatetags/i18n.py#L152. It should work the same like FilterExpression.resolve.

[Claude Paroz]

​https://github.com/django/django/pull/1988

[Claude Paroz <claude@…>]

In 02add43568b694ef25afeae474b5b1da883826c6:

Fixed #21417 -- Expanded TEMPLATE_STRING_IF_INVALID in blocktrans

Thanks keturn for the reporti, Chris Medrela for details and
Tim Graham for the review.
Refs #19915.