pull: make slightly clearer when failing for missing xattrs support
pull: make slightly clearer when failing for missing xattrs support

It is not clear why pull fails on tmpfs.  The additional message might
be helpful.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree