Throw ElementError when "Exit this page" button is missing

[romaricpascal]

Wording to match that of the Skip link component, but to be reviewed in #4072. Wording is now handled by the ElementError class and will match the other ElementError being thrown.

Closes #4129

[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	75.73 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	71.14 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.js	36.88 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.83 KiB

Modules

File	Size
all.mjs	67.24 KiB
components/accordion/accordion.mjs	21.78 KiB
components/button/button.mjs	4.58 KiB
components/character-count/character-count.mjs	21.52 KiB
components/checkboxes/checkboxes.mjs	5.28 KiB
components/error-summary/error-summary.mjs	5.88 KiB
components/exit-this-page/exit-this-page.mjs	16.47 KiB
components/header/header.mjs	3.21 KiB
components/notification-banner/notification-banner.mjs	4.42 KiB
components/radios/radios.mjs	4.29 KiB
components/skip-link/skip-link.mjs	3.63 KiB
components/tabs/tabs.mjs	8.95 KiB

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Action run for 26a179a

[colinrotherham]

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