We introduced new behavior into the driver to handle how 2.6 handles errors. However, this causes "warnings" to behave as errors in pre-2.6 servers. We need to revert this behavior when talking to pre-2.6 servers.

For example, when talking to a pre-2.6 server and performing a write that affects 0 documents while using a "w" field, the result is this:

WriteConcernException - WriteConcern detected an error 'no write has been done on this connection'. (Response was { "n" : 0, "lastOp" : { "$timestamp" : NumberLong(0) }, "connectionId" : 50820, "wnote" : "no write has been done on this connection", "err" : null, "ok" : 1.0 })

This causes an exception to be thrown, even though this is simply a warning.