Site broken after upgrading with Pathauto < 8.x-1.6
Based on the comments above from leymannx, I've manually uploaded Pathauto 1.6 to my site via FTP. Returning to /update.php, I got the option to update the database and that worked and my site is back up again. 

Note: The first time I tried this, I renamed the old Pathauto module folder to "pathauto_old", and uploaded the new version to a newly created folder just called "pathauto", and that did not work. update.php continued to report and out of date Pathauto. After that, I deleted both folders, re-uploaded pathauto 1.6, and then it worked.