More validation before queuing a cron job after pull failure

[nickdaugherty]

When a pull fails, more validation should be performed before re-queuing the job.

https://github.com/Automattic/syndication/blob/7149bc91471f66c05bfa7a744b8a08b31218da69/includes/class-syndication-site-auto-retry.php#L61-L67

Here, we need to verify that:

1. The post exists
2. The post is of the right type
3. The post has a valid-looking syndication url

This will prevent situations where cron events exist for invalid posts, but they keep requeuing indefinitely due to a failure to detect these conditions here.