More gnome-doc-utils breakage

[Shaun McCance <shaunm gmail com>]

The bg, nb, and no translations on HEAD all had broken XML.  There
was a missing left angle bracket on an ending tag, a missing slash
on an empty tag, and some malformed numeric entities.  I have fixed
the broken XML and committed to HEAD.

I've also added a well-formedness rule for l10n.xml to the Makefile.
Broken XML will now cause the build to break, so that I catch things
as soon as they happen.

--
Shaun