Throw errors during Tabs initialisation if key HTML elements are missing

[domoscargin]

Closes #4127

A coupla notes:

• So far, I've limited this PR to replacing empty returns in the initialisation of the component. There are 17 more empty returns in the component, some of which are bound during initialisation. These are in event handlers, however, which we can refactor as and when we add things to the API.
• I've removed a check in setup() as unnecessary. Some checks will need to replace it if setup() becomes an API method that can be called separately from initialisation, but I figure we'd probably do a slight code refactor then anyway.

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-4.6.0.min.css	118.07 KiB
dist/govuk-frontend-4.6.0.min.js	47.1 KiB
dist/govuk-frontend-ie8-4.6.0.min.css	79.27 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	77.65 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	72.94 KiB
packages/govuk-frontend/dist/govuk/all.mjs	3.84 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	120.45 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	37.89 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.83 KiB

Modules

File	Size
all.mjs	69.04 KiB
components/accordion/accordion.mjs	21.91 KiB
components/button/button.mjs	4.71 KiB
components/character-count/character-count.mjs	22.03 KiB
components/checkboxes/checkboxes.mjs	5.69 KiB
components/error-summary/error-summary.mjs	6.01 KiB
components/exit-this-page/exit-this-page.mjs	16.61 KiB
components/header/header.mjs	3.84 KiB
components/notification-banner/notification-banner.mjs	4.55 KiB
components/radios/radios.mjs	4.68 KiB
components/skip-link/skip-link.mjs	3.74 KiB
components/tabs/tabs.mjs	9.32 KiB

View stats and visualisations on the review app

---

Action run for 1377822

[colinrotherham]

🙌

[colinrotherham]

This is looking ace @domoscargin

Just merged #4261 so let's give this one a rebase and drop the [data-module="${Tabs.moduleName}"] prefix then think we're good to go 🚀

[colinrotherham]

✅ Approved

(One little comment, but only a quick tweak)