Cryptic TypeLoadException if actor proxy interface is non-public

Consider the following code snippet:

```csharp
ActorId actorId = new("123");
IMyActor proxy = ActorProxy.Create(actorId, nameof(MyActor));

// The Init method invokes the actor directly, which then decides whether to apply de-dupe logic.
await proxy.MyMethod();
```

Also, assume `IMyActor` is an internal interface (non-public).

## Expected Behavior

Internal interfaces should be supported, OR there should be a friendly exception explaining why my interface is invalid.

## Actual Behavior

If `IMyActor` is internal, I get the following cryptic exception:

> System.TypeLoadException: Type 'MyNamespace.IMyActor_.actorV1.proxyIMyActoractorV1Proxy' from assembly 'MyNamespace.IMyActor_.actorV1.proxy, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is attempting to implement an inaccessible interface.

It's not at all clear what needs to be done to fix this issue.