Issue a WARNING about invalid rule file format in contrib/unaccent.

We were already issuing a WARNING, albeit only elog not ereport, for
duplicate source strings; so warning rather than just being stoically
silent seems like the best thing to do here.  Arguably both of these
complaints should be upgraded to ERRORs, but that might be more
behavioral change than people want.

Note: the faulty line is already printed via an errcontext hook,
so there's no need for more information than these messages provide.