fixed unnecessary throwing of `DocumentKeyNotFound` exception

While profiling my app I saw a lot of exceptions thrown by reactivemongo, namely `DocumentKeyNotFound` and `ClassCastException` - actually I got **27500** `DocumentKeyNotFound`s and **10600** `ClassCastException`s in a very short period of time!

This pull request is a fix for the `DocumentKeyNotFound` exceptions. Instead of throwing it a `Failure` object will be created.