Fix error being printed to stdout

dotfiles/config/bashum/setup

 mark="# added by https://gitea.vornet.cz/netvor/mydots"
 if grep -qFe "$mark" "$HOME/.bashrc";
 then
-    echo "already set up, giving up";
+    echo >&2 "already set up, giving up";
     exit 1;
 else
     put ''