How to get an Error back from VB6.0 DLL?

I have a legacy VB6 DLL; we run it in COM+. We are now using it from a new C# service. How do I get the VB6 DLL to return an "exception" back to the C# service that would indicate a failure? I've tried Err.Raise and it will either a) throw up a message box and wait for END/DEBUG, or b) return without any error condition. I want to trap the error in a C# exception.

Thanks,
Joe