Adding the error to the translation means Rails will automatically show
the error message in the view.
We're also adding a test to make sure the error message is correctly
displayed.
We now have duplication in the validation rules, however. Validating
translatable attributes is still a bit of mess.
We don't need any row classes anymore because the <body> already has a
maximum width. As for columns, we only have one column in this form, so
we don't need them either. Besides, the form's parent element already
has a padding.
