Monetizable assignment

We where having an issue where users were setting currency inputs to "na" and money-rails was throwing an exception before validations had a chance to inform the user of their error (even with `[MoneyRails.raise_error_on_money_parsing]` set to `false`).

The simplest solution seems to be to capture `[Money::Currency::UnknownCurrency]` errors and only bubble them up if `[MoneyRails.raise_error_on_money_parsing]` is set to true.