Print PublicError traceback when in debug mode
    
    The framework only shows traceback for the internal/unknown errors,
    recognized PublicErrors are simply passed back to the FreeIPA
    clients.
    
    However, sometimes it would help to see a traceback of the
    PublicError to for example see exactly which line returns it.
    
    https://fedorahosted.org/freeipa/ticket/4847
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>