l10n_br_account_payment_boleto: No meaningful error message when partner address isn't complete

When the partner street number or street name are void you get tracebacks such as

```
File` "/workspace/parts/odoo-brazil-banking/l10n_br_account_payment_boleto/boleto/document.py", line 120, in _sacado
    self.boleto.sacado_endereco = partner.street + ', ' + partner.number
TypeError: coercing to Unicode: need string or buffer, bool found
```

A message for the user would be better