I also got an error when updating a client’s site. First time, too. The specific message was:
“The authenticity of WordPress-5.3-new-bundled.zip could not be verified as no signature was found.” with a message that the installation failed.
I checked the front end and the site seemed to be working (thankfully since it’s a high-traffic site).
I then went to the dashboard and was instructed to update the database (click) and did so. When I reloaded the dashboard the site was indeed working 5.3.
Anyway, it’s a mystery.