Currently DBClientConnection will sometimes return an internal code for network errors, which means its user cannot tell these are network errors which could be retried. We should change this so DBClientConnection and related classes pass any network errors up as network errors.

Examples:

https://github.com/mongodb/mongo/blob/master/src/mongo/client/dbclient_connection.cpp#L540

https://github.com/mongodb/mongo/blob/master/src/mongo/client/dbclient_connection.cpp#L625

https://github.com/mongodb/mongo/blob/master/src/mongo/client/dbclient_cursor.cpp#L276

https://github.com/mongodb/mongo/blob/master/src/mongo/client/dbclient_base.cpp#L591 (this is probably not a network error but we should still propagate the error code)

https://github.com/mongodb/mongo/blob/master/src/mongo/client/dbclient_base.cpp#L651

https://github.com/mongodb/mongo/blob/master/src/mongo/client/dbclient_base.cpp#L772

Some of these will require changing lower level classes to preserve error information.