TestReplicaSetClientAuth.test_auth_network_error fails frequently on the v2.8 branch, only against MongoDB 3+.

Likely MongoReplicaSetClient.__socket should catch all exceptions, not just OperationFailure, to return the closed socket to the pool before reraising:

https://github.com/mongodb/mongo-python-driver/blob/v2.8/pymongo/mongo_replica_set_client.py#L1309

But that alone does not fix the test, so there must be some other (additional?) cause.