Make config test exit with error code if broken

[jamesoff]

Per discussion with @cpina, make -t exit non-zero (I picked 2) if something isn't right with the config. Mostly this covers using a monitor/alerter/logger type which isn't known; other config errors currently will just throw an exception and exit 1 still.

If not testing the config, then the previous behaviour is currently retained: SimpleMonitor will try to continue with the config it did load. Not sure if that's the best choice yet; it probably should exit if the config is broken either way.

[cpina]

Idea/suggestion/nitpick: use something that is not exit code 2. Why? argparse, by default, exits when there is an invalid command line parameter with error code 2 (see https://docs.python.org/es/3/library/argparse.html#exit-on-error).

And since exit code 1 is for, usually, exceptions (IIRC) then use exit code 3 for invalid configuration?

In practical terms for systemd integration it doesn't matter, so I am happy either way!

[jamesoff]

Nice catch, 3 it is then :)