Add toggle to exit with error for missing dependencies

[atharvai]

Currently only Failed tasks results in luigi-monitor exiting with failed status code, This is a proposal to make luigi-monitor return a fail status code when there are missing dependencies. This can indicate some errors or undesired state.

Default behaviour should be the current behaviour and passing in toggle like
monitor(slack=SLACK_URL, fail_on_missing=True)
fail_on_missing is False by default.

[atharvai]

I spent some time on this today, but it looks like there isn't an easy or good way to capture the return status of luigi.run() from within the context manager.