vim-utils: better error message if duplicates

Print a more user friendly error message if you try to in install
multiple plugins with the same name
closes #145980 #146099

pkgs/misc/vim-plugins/vim-utils.nix

···
 
     linkVimlPlugin = plugin: packageName: dir: ''
       mkdir -p $out/pack/${packageName}/${dir}
+      if test -e "$out/pack/${packageName}/${dir}/${lib.getName plugin}"; then
+        printf "\nERROR - Duplicated vim plugin: ${lib.getName plugin}\n\n"
+        exit 1
+      fi
       ln -sf ${plugin}/${rtpPath} $out/pack/${packageName}/${dir}/${lib.getName plugin}
     '';