+ display lack of map in a nice way, don't generate bogus preview in the background * improve map selection behavior in online mode, so that players can load a missing map and have it correctly selected after pressing F5 without any rejoin, map-reselection. also doesn't cause "different maps generated" anymore if map is missing, instead the person lacking the map gets "invalid genmap" and drops out of game immediatly

[patterns]
**.h = native
**.c = native
**.cpp = native
**.lua = native
**.txt = native
**.pas = native
**.inc = native
**.hs = native