Hmm. Maybe I’m getting those messages confused now.
Yeah. I didn’t pay enough attention to the fact that it was an unsuccessful web upgrade. I suspect that this may be some other error having to do with the failed upgrade.
And now that I see that it was a couple days ago that the problem started, I guess this isn’t an emergency. 
Now that I’m noticing it was a failed web upgrade, I think the solution is
cd /var/discourse
git pull; ./launcher rebuild app