[1.0.X] Fixed #10014 -- Don't crash when using debug template tag inside a block node tag.

Returning non-ASCII characters from TextNode.__repr__ was causing
problems in the BlockNode.__repr__ method (and probably in other places
we don't know about yet). We now forcibly convert to ascii and replace any
unconvertible characters, rather than returning some moderately
corrupted data in the non-ASCII case.

Backport of r9757 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9758 bcc190cf-cafb-0310-a4f2-bffc1f526a37