Check if Dependency Resolver is configured to avoid a NullReferenceException

[augustoproiete]

When using the DI() extension methods on ActorSystem or IActorContext without having properly configured an IDependencyResolver we currently get a NullReferenceException (e.g. 1, 2).

using (var system = ActorSystem.Create("mySystem"))
{
    var actorRef = system.DI().Props<MyActor>(); // Throws NullReferenceException
    // ...
}

This PR adds a check on the constructors of DIActorSystemAdapter and DIActorContextAdapter to verify if a dependency resolver exists, and throws an InvalidOperationException with a clear message of what the problem is.

---

using (var system = ActorSystem.Create("mySystem"))
{
    var actorRef = system.DI().Props<MyActor>(); // Throws InvalidOperationException
    // ...
}

Unhandled Exception: System.InvalidOperationException:
The Dependency Resolver has not been configured yet

[Aaronontheweb]

Looks good to me. Much clearer than what we were doing before.