Add `full_error_message` wrapper.

This PR introduces the `full_error_message` wrapper, that can be configured to replace [the `:error`] [1] wrapper.

Also, I had to replace the `error` method inside the `User` class to use the `ActiveModel::Errors` so we can use the `full_messages_for(attribute)` method.

[1]: https://github.com/plataformatec/simple_form/blob/master/lib/generators/simple_form/templates/config/initializers/simple_form.rb#L45