I really dislike the attitude of "errors are rare, so don't spend much time on them" espoused by the article. Errors are rare in the sense that you will often miss them, but most of your users will run into them.

Let's say your AJAX requests have a .1% chance of failure. If your users perform a thousand actions each on average, then 50% of your users will have been exposed to your error flow. Hope it's better than "Sorry, an error occurred."

Individual errors are rare compared to successes. Overall errors happen all the time.