fail with warning if no active volumes are found

pandora_client/__init__.py

@@ -582,6 +582,10 @@ class Client(object):
         conn, c = self._conn()
 
         volumes = self.active_volumes()
+        if not volumes:
+            print "no active volumes found"
+            return
+
         for name in sorted(volumes):
             prefix = volumes[name]
             files = self.files(prefix)