pull-metalink: Don't print error output when we expect failure
pull-metalink: Don't print error output when we expect failure

I think I added `cat err.txt` to debug, but it's not necessary now.

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