The NoStreamingHTTPWorker should validate conversation configuration fields.

It seems the the worker choked on a conversation that had empty message_push, event_push urls. 

Here's the log of what happened:
https://gist.github.com/vgeddes/1bc8f87f2063fdb9205b

Until we fix this, we're just going to make sure that conversations are well-defined when used with this transport.