Missing group safeguards.

- Server will no longer start up when the guest or default groups cannot
  be located.
- Players joining with unknown groups assigned to them will be
  disconnected with an error

TShockAPI/TShock.cs

@@ -322,6 +322,7 @@ namespace TShockAPI
 				Regions = new RegionManager(DB);
 				UserAccounts = new UserAccountManager(DB);
 				Groups = new GroupManager(DB);
+				Groups.EnsureCoreGroupsPresent();
 				ProjectileBans = new ProjectileManagager(DB);
 				TileBans = new TileManager(DB);
 				RememberedPos = new RememberedPosManager(DB);