refs: Add g_prefix_error around opendir for easier debugging
refs: Add g_prefix_error around opendir for easier debugging

Addresses:
  https://github.com/projectatomic/rpm-ostree/issues/264

We should consider moving this down into `glnx_opendirat`, but for now
a quick fix.

Closes: #255
Approved by: jlebon

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