Stop sending empty changelogs on failure
    
    If there's a problem, the changelog will be empty, which is not
    particularly helpful to anybody. Instead of spamming devel@ list with
    that, just send the traceback to rel-eng@ together with version of
    compose-utils that was used.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>