localization: make clearer errors

I'm getting this error, and the gitlab log does not say which case is
failing.

tools/prepare-multi-lang.sh

@@ -98,7 +98,13 @@ if [ ! -f _config.yml ]; then
  exit 1
 fi
 
-if [[ $# == 0 || ! -d $1/en ]]; then
+if [[ $# == 0 ]]; then
+ echo "At least one argument required!"
+ exit 1
+fi
+
+if [ ! -d $1/en ]; then
+ echo "$1/en does not exist!"
  echo "First argument must be the directory of the Jekyll website."
  echo "This should take into account the baseurl as well. If the site"
  echo "is output to 'build/' and the baseurl is 'fdroid-website/' then"