It's complaining about gwPaperDollBagSlotButton1 not having a key called IconBorder. I can't see from the code supplied what you used to create gwPaperDollBagSlotButton1 (and presumably others).
My guess is you have something like:
Lua Code:
<Button name="gwPaperDollBag" ....>
<Button name="$parentSlotButton1" inherits="GwPaperDollBagItem" ...>
....
</Button>
<Button name="$parentSlotButton2" inherits="GwPaperDollBagItem" ...>
....
</Button>
<Button name="$parentSlotButton3" inherits="GwPaperDollBagItem" ...>
....
</Button>
...
</Button>
Where it should be:
Lua Code:
<Button name="gwPaperDollBag" ....>
<ItemButton name="$parentSlotButton1" inherits="GwPaperDollBagItem" ...>
....
</ItemButton>
<ItemButton name="$parentSlotButton2" inherits="GwPaperDollBagItem" ...>
....
</ItemButton>
<ItemButton name="$parentSlotButton3" inherits="GwPaperDollBagItem" ...>
....
</ItemButton>
...
</Button>
The base widget specified for creating the actual widget overwrites the base widget specified in the template it inherits.
Lua Code:
<Frame name="SomeTemplate" virtual="true">
...
</Frame>
<Button name="B1" inherits="SomeTemplate"> --creates a plain Button
<ItemButton name="B1" inherits="SomeTemplate"> --creates an ItemButton
Even though SomeTemplate is based on a Frame.