kivy build: test and document that make_locale is to be run first

contrib/make_apk

@@ -1,5 +1,22 @@
 #!/bin/bash
 
+set -e
+
+CONTRIB="$(dirname "$(readlink -e "$0")")"
+ROOT_FOLDER="$CONTRIB"/..
+PACKAGES="$ROOT_FOLDER"/packages/
+LOCALE="$ROOT_FOLDER"/electrum/locale/
+
+if [ ! -d "$LOCALE" ]; then
+  echo "Run make_locale first!"
+  exit 1
+fi
+
+if [ ! -d "$PACKAGES" ]; then
+  echo "Run make_packages first!"
+  exit 1
+fi
+
 pushd ./electrum/gui/kivy/
 
 make theming