CLI commands should exit-fail if no jobs scheduled
    
    Summary:
    Have the backing functions always raise TriggerExceptions if
    no jobs could be scheduled, and have the CLI command functions
    just quit (with exit code 1) immediately when this happens.
    I don't see any value to logging the errors and continuing to
    run, nothing useful is going to happen with no jobs.
    
    This allows us to have the 'current' cronjob run the compose
    report: we just have it do:
    
    openqa_trigger.py current && check-compose
    
    then it won't generate a compose report every time it runs, but
    only when the current compose changed and some jobs ran.
    
    Test Plan:
    Well, just make sure things quit or run as intended,
    I guess.
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D541