in case a calculation script is found, slice are not calculated in TradeModelLine class. This should be handle by the calculation script. This permit to create a customise behaviour for slices
- improve the error message in case of not found cells to make debugging easier


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@28027 20353a03-c40f-0410-a6d1-a30d3c3de9de