[FIX] Use the new product's UoM when checking for availability

Fixes #2559
This is fixes both the fact that suer-facing warning messages are wrong,
and the fact that qty_available gets computed with the wrong UoM, which
is a problem for custom addons - i.e. https://github.com/numerigraphe/stock-logistics-warehouse/commit/ad4a88088da6c40b253f6c014c57629df556c8b4