Fix runUpgrade tool issues
    
    Description: We should not lost any exception info.
    Change 'assert False' to 'raise'.
    
    Use sbin_dir parameter from defaults.inf instead of prefix.
    RHEL 6 has setup-ds.pl under /usr/sbin, and if we install
    Directory Server under '/' prefix current code would fail.
    Also, sbin_dir makes much more sence then prefix in this case.
    
    https://fedorahosted.org/389/ticket/49018
    
    Reviewed by: nhosoi (Thanks, Noriko!)