Follow up - I used the Joomlart Extension Manager to update another site to the latest T3 Framework, to 3.0.4 and received this error AGAIN.
I had to download the 3.0.4 from the download page.
This broken link/package in the Joomlart Extension Manager must be changed, or you will have many more people running into this problem.
I want to have confidence in the updater.
I will now update all my other sites manually.