server: Split out varargs version of wl_resource_post_error.

This will allow other wrappers around wl_resource_post_error to accept
variable argument lists.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>