Symfony version(s) affected: (at least) 4.1.2

Description
When enabling the Bootrap 4 form theme in the latest version of Symfony, validation errors are not displayed correctly*:

The error is displayed underneath the label instead of the actual widget. Fix: move the corresponding block to the correct position.

*) "correctly" means that is looks different from the original documentation here

How to reproduce

1. Configure twig to use the Bootrap 4 theme
2. Create a form and cause a validation error

Possible Solution
As suggested above.