fix for DuplicateKey Exception without code

We have some error handling code that depends on catching a DuplicateKey exception. We started getting a WriteConcernException instead after upgrading to the 2.11.3 version of the driver from 2.9.3

The problem is that mongoS doesn't return a "code" in the response (at least in the version which we're using:  2.2.3).  Previous versions of the driver checked for a string in the "err", but that was removed in 2.11.x.

 Here's an example of a response:

```json
{
    "err": "E11000 duplicate key error index: foursquare.coll.$_id_  dup key: { : 1 }",
    "errObjects": [
        {
            "code": 11000,
            "connectionId": 1514529,
            "err": "E11000 duplicate key error index: foursquare.coll.$_id_  dup key: { : 1 }",
            "lastOp": {
                "$inc": 9,
                "$ts": 1384877384
            },
            "n": 0,
            "ok": 1.0
        },
        {
            "code": 11000,
            "connectionId": 1866741,
            "err": "E11000 duplicate key error index: foursquare.coll.$_id_  dup key: { : 2 }",
            "lastOp": {
                "$inc": 0,
                "$ts": 0
            },
            "n": 0,
            "ok": 1.0
        }
    ],
    "errs": [
        "E11000 duplicate key error index: foursquare.coll.$_id_  dup key: { : 1 }",
        "E11000 duplicate key error index: foursquare.coll.$_id_  dup key: { : 2 }"
    ],
    "n": 0,
    "ok": 1.0,
    "serverUsed": "/127.0.0.1:27017",
    "shardRawGLE": {
        "shard0/a:27017,b:27017": {
            "code": 11000,
            "connectionId": 1514529,
            "err": "E11000 duplicate key error index: foursquare.coll.$_id_  dup key: { : 1 }",
            "lastOp": {
                "$inc": 9,
                "$ts": 1384877384
            },
            "n": 0,
            "ok": 1.0
        },
        "shard1/c:27017,d:27017": {
            "code": 11000,
            "connectionId": 1866741,
            "err": "E11000 duplicate key error index: foursquare.coll.$_id_  dup key: { : 2 }",
            "lastOp": {
                "$inc": 0,
                "$ts": 0
            },
            "n": 0,
            "ok": 1.0
        }
    },
    "shards": [
        "shard0/a:27017,b:27017",
        "shard1/c:27017,d:27017"
    ]
}
```