Better Error Handling when Connecting to Non-existent keyspace

No problem! This is running clean on Jenkins and has been merged to DSE here as well. Moving to done.

My mistake for not moving this to Testing earlier. I think this is done but I'll let Jaume do the honors in case I missed something.

Tests to this where added here.

To summarize some internal discussion: there was some discussion of putting this off to 4.0, but we decided that wasn't necessary. There's an argument that we shouldn't break the Session._init_ API in 3.9, but the Session docs explicitly say it's not to be used directly. Given that, it's worth breaking that API to fix the "Cluster.connect can return without being able to execute anything" bug.