Socket leak on exhaust query failure

If the server returns an error when an exhaust cursor makes its initial query, a socket is leaked and the max_pool_size semaphore is never incremented:

collection.find({'$bad_query_operator': 1}, exhaust=True)

The bug is in both MongoClient and MongoReplicaSetClient.

A network error on an exhaust cursor's initial query, on the other hand, is handled correctly in MongoReplicaSetClient, but causes a leak in MongoClient.