Change error notification for unknown nodes fail

Before, notification was "Deployment has failed. Check these nodes:
Unknown error".

Now it is more comprehensible. Information about failed nodes is coming
in one of next tasks, so we let the user know that.

Change-Id: I0f397b37747ba654823a819cc957ff6fad450ba1
Partial-Bug: #1506008

nailgun/nailgun/rpc/receiver.py

@@ -463,7 +463,8 @@ class NailgunReceiver(object):
             else:
                 message = u"\n".join(nodes_info)
         else:
-            message = u"Unknown error"
+            message = (u"Unknown nodes. Please wait for master task to "
+                       u"report which nodes were affected")
         return message
 
     @classmethod