I took a look at the update zips of WordPress 5.1 and 5.2, and it looks like the archive for WordPress 5.2 is slightly bigger than the 5.1 version. Annoyingly, it looks like this small size increase caused the archive size to go just over 10 million bytes (10 MB). This probably caused the file size limit to kick in and mess up the WordPress update.
Given the popularity of WordPress, this is probably going to become a substantial problem in the future. So I will check if there is some way to ensure WordPress updates will stay quick and easy.
But if you would like to upgrade right now, your best bet is probably to do the upgrade by hand. WordPress has step by step instructions detailing how to do that: Updating WordPress « WordPress Codex