Provide default PartialFunctions to recover from silhouette's exceptions

Silhouette defines 2 exceptions 
AuthorizationException
AccessDeniedException 

Recovering the failed futures which contain these exceptions would be simplified by having a default partial function for recovery offering a meaningful mapping from the Exception to an HTTP Code 
AuthenticationException should likely be mapped to 401 Unauthorized
AccessDeniedException should likely be mapped to 403 Forbidden