ignoring the Packages folder that is created in unity-environment for Unity version 2017.3

Print a message if someone tries to lauch ppo with the load flag but an invalid run-path

.gitignore

 /unity-environment/[Oo]bj/
 /unity-environment/[Bb]uild/
 /unity-environment/[Bb]uilds/
+/unity-environment/[Pp]ackages/
 /unity-environment/Assets/AssetStoreTools*
 
 # Tensorflow Model Info

python/ppo.py

     if load_model:
         print('Loading Model...')
         ckpt = tf.train.get_checkpoint_state(model_path)
+        if ckpt == None:
+          print('The model {0} could not be found. Make sure you specified the right '
+            '--run-path'.format(model_path))
         saver.restore(sess, ckpt.model_checkpoint_path)
     else:
         sess.run(init)