Raise MultipleFileFoundError.

If the directory has more than one entry, the lib must raise such Error.

slapos/libnetworkcache/__init__.py

@@ -163,7 +163,16 @@ class NetworkcacheClient(object):
     if result.status != 200:
       raise DirectoryNotFound(result.read())
 
-    return json.loads(data)
+    data_list = json.loads(data)
+    if len(data_list) > 1:
+      raise MultipleFileFoundError('Too many entries for a given directory. ' \
+               'Directory: %s. Entries: %s.' % (directory_key, str(data_list)))
+
+    return data_list
+
+
+class MultipleFileFoundError(Exception):
+  pass
 
 
 class DirectoryNotFound(Exception):