Feast serving URL should not be required

[woop]

Currently the Python SDK requires the Feast Serving URL to be defined when calling the version() method. The failure here should be graceful.

ValueErrorTraceback (most recent call last)
<ipython-input-31-b8a3b34abdf1> in <module>
----> 1 client.version()

/opt/notebooks/feast/sdk/python/feast/client.py in version(self)
    105 
    106         self._connect_core()
--> 107         self._connect_serving()
    108 
    109         core_version = ""

/opt/notebooks/feast/sdk/python/feast/client.py in _connect_serving(self, skip_if_connected)
    179         if self.__serving_channel is None:
    180             self.__serving_channel = grpc.insecure_channel(self.serving_url)
--> 181 
    182         try:
    183             grpc.channel_ready_future(self.__serving_channel).result(

ValueError: Please set Feast Serving URL.