Deleting the folder for a session and then trying to restore that session breaks

Testing #333370

I'm not sure if this is specific to migration, but I tried creating a CLI session in a folder, then going through the migration steps. Before I restarted, I deleted the folder, so when I would click the session in the agents window, I wanted to see what would happen if the folder no longer existed. It showed the session for a second, and then it disappeared and took me back to the new sessions page with that folder selected. We should have an error message showing that the folder has been deleted so the user knows what's happened.