Use Future error handling instead of the additional Try type

As noticed by @jeantil in the [Future[Try[T]] api signatures](https://groups.google.com/d/msg/play-silhouette/5apL4RzVPpQ/9zNq1ROFHwsJ) thread on the mailing list, the additional Try bloats the client code and the Silhouette provider implementations. Furthermore it isn't necessary to use an additional error handling layer on top of a Future, because a Future has its own error handling by the usage of Try.