repo: Improve error handling in sign_data()
repo: Improve error handling in sign_data()

Use _ostree_gpg_error_to_gio_error() so the actual GPG error message is
included in the GError.  Then apply an "Unable to blah: " message prefix.

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