PRINCE: gen-translations.sh expects an input filename now

This script used to reference a submodule in the ScummVM source tree
with the translation sources.
That submodule was removed, so the script now expects an argument and
prints a hint if the directory does not exist:

```
% ./engines/prince/gen-translations.sh
Usage: ./engines/prince/gen-translations.sh <path-to-scummvm-game-translations>
Download or clone from https://github.com/scummvm/game-translations/tree/prince-and-the-coward
```

Author: Botje

engines/prince/gen-translations.sh

@@ -1,6 +1,15 @@
 #! /bin/bash
 
-cp -v ../../../scummvm/devtools/create_prince/en.po .
+set -e
+
+translations_dir="$1"
+if [ -z "$translations_dir" ] || [ ! -d "$translations_dir" ]; then
+  echo "Usage: $0 <path-to-scummvm-game-translations>" >&2
+  echo "Download or clone from https://github.com/scummvm/game-translations/tree/prince-and-the-coward" >&2
+  exit 1
+fi
+
+cp -v "$translations_dir"/en.po .
 perl po-parse.pl en en.po
 ../../scummvm-tools-cli --tool pack_prince .
 cp -v prince_translation.dat ../../../scummvm/dists/engine-data/