add new error banner module for showing users js errors

this ensures it's reusable by other components, and easier to unit test
by isolating the separate concerns

note: this is not in Modules since that's designed for classes that are
then bound to an element in the DOM as indicated by a data-module
attribute. This will just live at the window.GOVUK level since we want
there to only ever be one `.banner-dangerous` warning.

app/assets/javascripts/errorBanner.js

@@ -0,0 +1,18 @@
+(function (window) {
+  "use strict";
+
+  /*
+  This module is intended to be used to show and hide an error banner based on a javascript trigger. You should make
+  sure the banner has an appropriate aria-live attribute, and a tabindex of -1 so that screenreaders and keyboard users
+  are alerted to the change respectively.
+
+  This may behave in unexpected ways if you have more than one element with the `banner-dangerous` class on your page.
+  */
+  window.GOVUK.ErrorBanner = {
+    hideBanner: () => $('.banner-dangerous').addClass('govuk-!-display-none'),
+    showBanner: (errorMessage) => $('.banner-dangerous')
+      .html(`<span class="govuk-visually-hidden">Error:</span> ${errorMessage}`)
+      .removeClass('govuk-!-display-none')
+      .trigger('focus'),
+  };
+})(window);