Make missing translations raise exceptions.

Discussed as preferable to #819 

This fails a bunch of tests that we'll need to get fixed before we merge this.

I'll be following those up. At a glance, these are missing:

```
cy.home.show.view_all_link_expanded
en.affordability.activemodel.attributes.mortgage_calculator/person_one.annual_income
cy.affordability.activemodel.attributes.mortgage_calculator/person_one.annual_income
en.debt_free_day_calculator.debts.form.required
cy.debt_free_day_calculator.debts.form.required
```