Handling of absent tables

Context: from time to time I work on a minimal total conversion template. At some point in 23.0's dev cycle it was in a launchable state, but subsequent changes to the asteroids and ranks table handling have made it unlaunchable because of missing data. possibly more, since I haven't resolved those cases yet.

In some cases this extra validation is probably the best way to go. When practical, I think it'd be preferable to let a TC omit tables controlling systems they don't intend to use. Unfamiliarity with these systems means I don't have any idea which bin these two cases fall in.