For what its worth, I reverted my VM a few times and retried this upgrade 2 different ways:
Way #1 - Do the upgrade, and then do a rebuild right after the error.
Way #2 - Remove discourse-data-explorer plugin from app.yml, rebuild, re-add discourse-data-explorer plugin to app.yml, rebuild.
Both seemed to work fine. I’ll also note that the above rebuilds also have the effect of upgrading the install to 3.1.0.beta3. However, the error in Way #1 has me nervous that some file or data might get left in an non-optimal state due to the interrupted upgrade. So, I’ll likely do Way #2 in production.