I've had some issues with this in the past.
You could try the filter 'Member of SFDC Campaign' and use 'Not In' and define your campaign.
Not sure how to do a real-time alert. I had attempted the API and reading Activities in the past for this, but the system was not returning the activity details when there is an error on the SFDC side.
We're trying to figure out something similar. We've noticed that, on occasion, a lead will fail to run through our central "sync to CRM" trigger campaign (via a "request campaign" flow step). Since Marketo doesn't re-run failed flow steps - just failed sync attempts - we need a way to identify this so that we can manually fix these when they happen.