[reactive-mongo] Manage success and error case in Response factory.

```scala
import scala.util.Try
import reactivemongo.bson.BSONDocument
import reactivemongo.core.protocol.Response
import acolyte.reactivemongo.MongoDB

val error: Try[Response] = MongoDB.Error(1/* channel */, "Error message")

val success: Try[Response] = MongoDB.Success(2/* channel */,
  BSONDocument("prop" -> "doc1"), BSONDocument("prop" -> "doc2")/*...*/)
```