Hello!
Full duplex doesn't have anything to do with it, you need to add the
serviceDebug behavior to your service and set includeExceptionDetailInFaults
equal to true. Just be warned that including exception information could
cause a security risk depending on your scenario. That will allow you to see
the exception information that occurred at the service. As for bubbling the
information up to the UI, we can't help you with that. It depends entirely
on your application and how you consume the services.
Here's a link explaining the setting:
http://msdn.microsoft.com/en-us/library/ms788993.aspx
Good luck!