Tweak rng-schema-locating-files directly, not in a hook function

This also makes me uncomfortable - tweaking a custom variable instead of a normal variable -
but at least it saves us from having to use a hook function.  With this,
nxml-mode *will* set the Mallard schema and validate even the first XML file you open
in an Emacs session.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>
Tell the user if something goes wrong while loading the Mallard schema

Signed-off-by: Federico Mena Quintero <federico@gnome.org>