In addition to Arun's response, AA can only capture data when the page is online and functioning, therefore, if you need to track when its offline, you will need an additional resource that will send that data to Adobe as Adobe won't fire by default.
So for example if you have a soft error, say 400 or 500, you could have those pages send data to Adobe which you can use to determine uptime/downtime of all your pages.
If you have a hard failure with no response to the user, you will need to put some mechanism in place on the back-end that will detect that and send the data to Adobe manually or you can create a data feed from your logs and upload.
