HRESULTs are COM's weakness, error codes don't scale well. At least part of the reason that Java ate Microsoft's lunch.
The error code doesn't mean anything more than "could not attach debugger", there isn't any room to also unequivocally explain why it couldn't be attached. Nothing similar to, say, an InnerException. So you just get a diagnostic for what couldn't be done, not a diagnostic for why it couldn't be done. It is as good as its going to get, maybe they'll implement IErrorInfo some day so you'll at least get a textual description. More likely is that the Visual Studio extension model is going to get a major overhaul first. Which is the way the wind is blowing, add-ins were deprecated in VS2013.