Also:
- Check if an exchange rate is defined for the selected currency
- Check if the amount is defined (was done for puchases only)
- Check amounts consistency before outgoing inventory

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@38977 20353a03-c40f-0410-a6d1-a30d3c3de9de