show error message in banner rather than an alert

the banner is a nicer user experience, and consistent with how we
display errors elsewhere in notify. For now pass through the error
message from JS, but we'll probably want to change that since the erorr
messages themselves are often a bit cryptic and unhelpful

app/assets/javascripts/registerSecurityKey.js

@@ -7,6 +7,9 @@
         .on('click', function(event) {
           event.preventDefault();
 
+          // hide any existing error prompt
+          window.GOVUK.ErrorBanner.hideBanner();
+
           fetch('/webauthn/register')
             .then((response) => {
               if (!response.ok) {
@@ -44,9 +47,9 @@
             .catch((error) => {
               console.error(error);
               // some browsers will show an error dialogue for some
-              // errors; to be safe we always pop up an alert
-              var message = error.message || error;
-              alert('Error during registration.\n\n' + message);
+              // errors; to be safe we always display an error message on the page.
+              const message = error.message || error;
+              window.GOVUK.ErrorBanner.showBanner('Something went wrong');
             });
         });
     };