I can't play the game anymore since I've started getting this error every time I launch it. It worked just fine earlier in the day.
As the error message suggests exiting the game and quitting Steam completely, I've both restarted Steam and my computer to see if it would help, but it didn't fix it.

Edit: The problem has gone away overnight. I assume it may have been tied to an increase in Steam downtime reports around the time, but I couldn't find any official information on the issue.