Terminated ajax requests throw error even if terminated because of a form submit

Problem/Motivation

There was a similar thread for D7 - #2808789: Fix "An AJAX HTTP request terminated abnormally" alert after user has navigated away from the page.

The problem we are experiencing is mostly in JS functional tests having the autosave_form module enabled where ajax request are triggered at a specific interval but the form is submitted before an ajax request has finished, in which case the ajax request is terminated and drupal is throwing a JS error, at which point phantomjs will interrupt the test and exit with an error.

Proposed resolution

Do not throw the error if the response has the status code 0 and the unbeforeunload event has been triggered.

Remaining tasks

User interface changes

API changes

Data model changes