DCAwareRoundRobinPolicy: Raise an error for an unconfigured DC

In the next major and as part of PYTHON-811, specifying a DC aware LBP without also specifying an explicit DC will result in an exception (or warning? The debate isn't clear yet). Make sure error messages for an unconfigured DC using a DC aware policy actually show the available DCs, and possibly a snippet for how to configure it in code.