fail when there are borders that can't be read because their endpoints are missing

scripts/kill-planes.lua

@@ -21,3 +21,5 @@ for k,v in ipairs(ids) do
     p:erase()
 end
 eressea.write_game(get_turn() .. '.new')
+eressea.free_game()
+eressea.read_game(get_turn() .. '.new')

src/kernel/connection.c

@@ -633,8 +633,8 @@ int read_borders(gamedata *data)
             from = findregionbyid(fid);
             to = findregionbyid(tid);
             if (!to || !from) {
-                log_warning("%s connection between incomplete regions %d and %d", zText, fid, tid);
-                continue;
+                log_error("%s connection between missing regions %d and %d", zText, fid, tid);
+                assert((to && from) || !"connection between missing regions");
             }
         }