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.

syndication/includes/class-syndication-site-auto-retry.php

Lines 61 to 67 in 7149bc9

	$site = get_post( $site_id );
	// Fetch the site url
	$site_url = get_post_meta( $site->ID, 'syn_feed_url', true );
	// Fetch the number of times we've tried to auto-retry
	$site_auto_retry_count = (int) get_post_meta( $site_id, 'syn_failed_auto_retry_attempts', true );

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.