TSG S04 Fix bug: Separate undead comments

Deoran and Ethiliel's comments should apply when there are undead remaining. This is separate to a general command about arrows (pierce attacks) not working on skeletons.

data/campaigns/The_South_Guard/scenarios/04_Vale_of_Tears.cfg

@@ -278,19 +278,35 @@
 
         # only comment on pierce damage being useless if the player didn't
         # actually kill the skeleton with it
-        [if]
+        [filter_condition]
             [variable]
                 name=second_unit.hitpoints
                 greater_than=0
             [/variable]
+        [/filter_condition]
 
-            [then]
-                [message]
-                    speaker=unit
-                    message= _ "Our weapons are useless against these skeletons! They have nothing for our shafts to pierce but air!"
-                [/message]
-            [/then]
-        [/if]
+        [message]
+            speaker=unit
+            message= _ "Our weapons are useless against these skeletons! They have nothing for our shafts to pierce but air!"
+        [/message]
+    [/event]
+
+    [event]
+        name=attack_end
+
+        [filter]
+            side=1
+        [/filter]
+
+        [filter_second]
+            side=2
+        [/filter_second]
+
+        [filter_condition]
+            [have_unit]
+                side=2
+            [/have_unit]
+        [/filter_condition]
 
         [message]
             speaker=Deoran