core: Add context when we fail to chmod()/chown()
core: Add context when we fail to chmod()/chown()

This helps make things less confusing when we try to create a file not
owned by our uid, such as running ostree-pull as non-root on a
repository owned by root.

leandrosansilva/ostree,gatispaeglis/ostree,leandrosansilva/ostree,resin-io/ostree,GNOME/ostree,leandrosansilva/ostree,gatispaeglis/ostree,sujitfulse/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,resin-io/ostree,sujitfulse/ostree,resin-io/ostree,GNOME/ostree,GNOME/ostree,sujitfulse/ostree,resin-io/ostree,GNOME/ostree,GNOME/ostree,sujitfulse/ostree,leandrosansilva/ostree,resin-io/ostree,sujitfulse/ostree,leandrosansilva/ostree