eclipse: fix update script to fail on prefetch failures

            url="https://www.eclipse.org/downloads/download.php?r=1&nf=1&file=/technology/epp/downloads/release/${year}-${month}/R/eclipse-${id}-${year}-${month}-R-linux-gtk-${arch}.tar.gz";

        fi

        echo "prefetching ${id} ${arch}";

        # sometimes a mirror is down; retrying a few times should eventually get us redirected to a working mirror

        for try in $(seq 1 5); do

            echo "prefetching ${id} ${arch} (try ${try})";

            h=$(nix store prefetch-file --json "$url" | jq -r .hash);

            if [ "$h" != "" ]; then break; fi

        done

        if [ "$h" == "" ]; then

            echo "unable to prefetch and hash ${id} for ${arch} from ${url}";

            echo "see above output for errors";

            exit 1;

        fi

        t=$(mktemp);

        cat $ECLIPSES_JSON | jq -r ".eclipses.${id}.hashes.${arch} = \"${h}\"" > $t;

        mv $t $ECLIPSES_JSON;