[FIX] stock: change error message

The error can be raised either when a move is in state done or cancel. However, the error message only says we cannot split split if the move is in done, which can be misleading.

closes odoo/odoo#132389

X-original-commit: 45bab967
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Pedro Nogueira (pno) <pno@odoo.com>