provision: Give concrete NFS error message on older OSX versions.

tools/setup/generate_zulip_bots_static_files

@@ -17,7 +17,14 @@ from zulip_bots.lib import get_bots_directory_path
 bots_dir = os.path.join(settings.STATIC_ROOT, 'generated/bots')
 if os.path.isdir(bots_dir):
     # delete old static files, they could be outdated
-    shutil.rmtree(bots_dir)
+    try:
+        shutil.rmtree(bots_dir)
+    except OSError:
+        print("There was a permission error with Vagrant's NFS file sharing on old versions of " +
+              "OSX. You can fix this by upgrading to the latest OSX High Sierra, or work around it " +
+              "by giving read/write permissions to your /zulip folder to everyone.")
+        sys.exit(1)
+
 
 os.makedirs(bots_dir, exist_ok=True)