[1.2.X] Fixed #15502 -- Ensure that nested TemplateDoesNotExist errors are...

[1.2.X] Fixed #15502 -- Ensure that nested TemplateDoesNotExist errors are propegated with a meaningful error message when loaded using select_template. Thanks to jaylett for the report, and GDorn for the patch.

Backport of r15717 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15718 bcc190cf-cafb-0310-a4f2-bffc1f526a37