Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
Couldn't you just raise the exception here:
```python
if not (
    line.move_line_id and line.move_line_id.invoice
    and line.move_line_id.invoice.vat_on_payment
):
```
Then do away with `vat_on_p` and return `valid_lines`