...return value from ToolBox::GetItemId, as is done in all other places where
such is returned from one of the ToolBox::GetItemId overloads.  And
TOOLBOX_ITEM_NOTFOUND is otherwise only used for item positions, not item IDs.
(This lone odd use of TOOLBOX_ITEM_NOTFOUND was introduced with
ae5f75ab "Allow a common op. on a disposed
toolbox", probably by accident.)

Change-Id: Id767b1d8d0b3cdfdc0ec525f165d75ae0cef7301