core: Print which xattrs are not sorted

core: Print which xattrs are not sorted

I hit this in the rpm-ostree rechunker flow somehow
and with this change I now get:

`error: Generating commit from rootfs: Processing dir var: Writing content object: Incorrectly sorted xattr name (prev=user.Librepo.checksum.mtime, cur=security.selinux), index=2`

which is more useful, although I still need to figure out
and fix why that's happening (again?).

Signed-off-by: Colin Walters <walters@verbum.org>