Log ActorInitializationException instead of underlying exception

We currently log the underlying exception of an `ActorInitializationException` since I screwed up porting this from AKKA JVM a week back. :)
We should only use the message from the underlying exception, and still log the entire `ActorInitializationException`.

This is important since it's otherwise impossible to test that `ActorInitializationException` is thrown.
This was changed since calling `public ActorInitializationException(string message, Exception cause = null)` is handled by `public ActorInitializationException(string message)`.