Move navmesh warning from `msg` to `dirty`.

... for `ThievesGuildTrapdoor.esp` as per the mod author's comments:
- http://forums.bethsoft.com/topic/1506584-rel-loot-thread-12/?p=23769407
- http://forums.bethsoft.com/topic/1506584-rel-loot-thread-12/?p=23769427
Add `url` for `ThievesGuildTrapdoor.esp`.

Using Nexus one; it's also available on Steam:
http://steamcommunity.com/sharedfiles/filedetails/?id=8832