Throw MissingElementError when "Exit this page" button is missing

Wording to match that of the Skip link component, but to be reviewed in #4072.

Closes #4129

packages/govuk-frontend/src/govuk/components/exit-this-page/exit-this-page.mjs

@@ -1,5 +1,6 @@
 import { mergeConfigs, extractConfigByNamespace } from '../../common/index.mjs'
 import { normaliseDataset } from '../../common/normalise-dataset.mjs'
+import { MissingElementError } from '../../errors/index.mjs'
 import { GOVUKFrontendComponent } from '../../govuk-frontend-component.mjs'
 import { I18n } from '../../i18n.mjs'
 
@@ -86,7 +87,7 @@ export class ExitThisPage extends GOVUKFrontendComponent {
 
     const $button = $module.querySelector('.govuk-exit-this-page__button')
     if (!($button instanceof HTMLElement)) {
-      return this
+      throw new MissingElementError('Exit this page: Button not found')
     }
 
     this.config = mergeConfigs(

packages/govuk-frontend/src/govuk/components/exit-this-page/exit-this-page.test.js

@@ -208,6 +208,20 @@ describe('/components/exit-this-page', () => {
           message: 'GOV.UK Frontend is not supported in this browser'
         })
       })
+
+      it('throws when the button is missing', async () => {
+        await expect(
+          renderAndInitialise(page, 'exit-this-page', {
+            params: examples.default,
+            beforeInitialisation($module) {
+              $module.querySelector('.govuk-exit-this-page__button').remove()
+            }
+          })
+        ).rejects.toEqual({
+          name: 'MissingElementError',
+          message: 'Exit this page: Button not found'
+        })
+      })
     })
   })
 })