Explain unresolvable references better

[odersky]

We run into problems when referring to a member of a self type of a class that it not also a member of the class from outside via an asSeenFrom.

One example is in 11226.scala where we see:

trait ManagedActorClassification { this: ActorEventBus =>
  def unsubscribe(subscriber: Subscriber): Unit
}

class Unsubscriber(bus: ManagedActorClassification) {
  def test(a: ActorRef): Unit = bus.unsubscribe(a) // error
}

The problem is that unsubscribe refers to the type Subscriber which is not resolvable as a member of bus. one idea could be to rule out type signatures like unsubscribe, similar how we rule out public signatures referring to private members. But this could rule out existing valid programs. For instance, the unsubscribe signature is unproblematic if it gets only called with prefixes that inherit ActorEventBus. You could say that the problem was instead that the type of bus was not specific enough.

In the long term, maybe restricting the signature is the right move. But for now, we just try to give better error messages in the case of existing failures.

Fixes #11226

[odersky]

@jchyb I had to add another test to the BestEffortCompilation blacklist. Here the compiler survives a MissingType exception in Typer but continuing leads to the same exception being thrown without a handler that reports an error.