prefer the venv pybabel but allow the system one to be used (if found)

etc/translation.vars

@@ -4,3 +4,12 @@ BABELCFG=etc/babel.cfg
 POTDIR=pots
 PROJDIR=i2p2www
 TRANSDIR=$PROJDIR/translations
+
+if [ -x env/bin/pybabel ]; then
+    PYBABEL=env/bin/pybabel
+elif [ $(which pybabel) ]; then
+    PYBABEL=$(which pybabel)
+else
+    echo "ERROR: pybabel was not found. Please run setup_venv.sh" >&2
+    exit 1
+fi