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.
This doesn't work on my machine, I had to use string format.