hard-code html error message for errorBanner

turns out that we're only using errorBanner with a static message, and
it's also full of rich html content. This means that it's probably
better to put it in the html templates with other content, rather than
hidden away in js files if we can help it.

Since there are two places, had to dupe the error message but i think
that's fine as i don't anticipate this error message being used in
significantly more places.

making it a string is a bit gross and means we don't get nice syntax
highlighting on it, but as it needs to be passed in to a jinja macro
that's the way it has to go unfortunately.

tests/javascripts/errorBanner.test.js

@@ -30,10 +30,6 @@ describe("Error Banner", () => {
       window.GOVUK.ErrorBanner.showBanner('Some Err');
     });
 
-    test("Will set a specific error message on the element", () => {
-      expect(document.querySelector('.banner-dangerous').textContent).toEqual('Error: Some Err')
-    });
-
     test("Will show the element", () => {
       expect(document.querySelector('.banner-dangerous').classList).not.toContain('govuk-!-display-none')
     });