Bail after failed compose in nightly-modular.sh
    
    When nightly-modular.sh was added, it wasn't set to check the
    exit code from pungi-koji, as nightly.sh does. This was I guess
    because it didn't do much after the compose finished at that
    time, and the author thought it was OK for the few things it
    *did* do to happen on both successful and failed composes.
    
    However, it now does more stuff after compose completion, like
    nightly.sh, including trying to generate changelog files and
    send out a compose report email. But because the pungi-koji
    exit code check is missing, it tries to do these things even
    for failed composes. Changelog generation will fail when the
    compose failed, and the email that gets sent out will have a
    missing compose ID (because NEWCOMPOSE_ID and SHORTCOMPOSE_ID
    can't be set properly and will be blank), and will be empty
    because the changelog file that's supposed to be read in as the
    content of the email isn't there. This is why we are getting
    an empty mail with subject "Fedora Modular bikeshed compose
    report:  changes" every time a compose fails.
    
    So, let's put that check in just like nightly.sh does.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>