SparkClientImpl should react to errors sent from the RemoteDriver

Right now, when RemoteDriver#shutdown runs it may / may not send an error message to the client.

When it does, we simply log the error and then proceed continue running normally. Eventually we will hit an error saying that the RPC channel has been closed.

There are two improvements we can make:
(1) If we get an error from the RemoteDriver we can treat the RPC channel as functionally closed, rather than proceeding with normal execution.
(2) If there is no known error on the driver side, at least send some type of termination signal so the client knows a shutdown is coming and can act accordingly.