By default message is looked up by I18n

ActiveRecord has an elaborate system of looking up translactions for error messages from default to more specific ones. http://guides.rubyonrails.org/i18n.html#basic-lookup-scopes-and-nested-keys

```
  activerecord:
    errors:
      messages:
        invalid: "Can you take another look here?"
      models:
        lead_form:
          attributes:
            redirect_url:
              invalid: "Ooops! This thingy doesn't look like a proper URL"
```

Default message ```:invalid``` makes ActiveRecord look up translations while still allowing message to be overridden in the model.