respec pill (error)

Description of problem

Clicking on the error pill does not show a list. This goes wrong when the error is a string.

When the error is a string then firstElementChild and lastElementChild are both null, firstChild and lastChild contain text.
The part in ui.js that wants to show the list expects html (from objects) and should have an additional check.

  if (fragment.firstElementChild === fragment.lastElementChild) {

    li.append(
      .../** @type {Element} */ (fragment.firstElementChild).childNodes
    );

An alternative approach could be to get rid of string errors/warnings.

URL to affected spec or repo: https://geonovum.github.io/dso-cim-ow/

What happened (e.g., it crashed)?: Code crashes (developer console)

Expected behavior (e.g., it shouldn't crash): When clicking on the pill it should show a list (even for string messages).

Optional, steps to reproduce:

1. leave out the mandatory sotd section (SotD paragraph).
2. open the document and click on the red pill
3. no list is shown