Managed Debugging Assistants get reset whenever the button is toggled

It seems that

``` csharp
            var guid = Guid.Empty;
            var hr = Session.RemoveAllSetExceptions(ref guid);
```

resets state of MDAs (and potentially other not-.NET exceptions?).

Since setting exceptions without `RemoveAllSetExceptions` does not work for some mysterious VS/COM reason, I'll have to record state of MDA, and restore it after the Remove call.
