Fixed #5124 -- Added a reasonable error when "extends" is not the first...

Fixed #5124 -- Added a reasonable error when "extends" is not the first template tag. Patch from k0001.

Refs #6274.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7084 bcc190cf-cafb-0310-a4f2-bffc1f526a37