download: check value that is returned by buildout downloader

slapos/recipe/download.py

@@ -73,9 +73,11 @@ class Recipe(object):
 
   def _download(self):
     alternate = self._alternate
-    download.Download(self._buildout, hash_name=True)(
+    path, is_temp = download.Download(self._buildout, hash_name=True)(
       self._url, self._md5sum, self._destination,
       **({'alternate_url': alternate} if alternate else {}))
+    assert path == self._destination and not is_temp, (
+      "SlapOS buildout >= 2.7.1+slapos014 needed")
 
   def update(self):
     if not self._md5sum: