MongoReplicaSetClient must raise error if no members of host list are reachable

If a replica set config contains hostnames that are not publicly addressable, e.g. "internal1, internal2, ...", and MongoReplicaSetClient connects to one using its public hostname, e.g. "example.com", it should raise a ConnectionFailure even though its initial connection to example.com succeeds.

However, in PyMongo 2.6.3 MongoReplicaSetClient.__init__ doesn't raise. It returns a client connected to no members, which is useless.