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/templates/views/two-factor-webauthn.html

@@ -21,6 +21,15 @@
 
 {% block maincolumn_content %}
 
+    {{ govukErrorMessage({
+      "classes": "banner-dangerous govuk-!-display-none",
+      "text": "There was a javascript error.",
+      "attributes": {
+        "aria-live": "polite",
+        "tabindex": '-1'
+      }
+    }) }}
+
   <div class="govuk-grid-row">
     <div class="govuk-grid-column-one-half">
       {{ page_header(page_title) }}

app/templates/views/user-profile/security-keys.html

@@ -46,6 +46,16 @@
 {% endset %}
 
   <div class="govuk-grid-row">
+
+    {{ govukErrorMessage({
+      "classes": "banner-dangerous govuk-!-display-none",
+      "text": "There was a javascript error.",
+      "attributes": {
+        "aria-live": "polite",
+        "tabindex": '-1'
+      }
+    }) }}
+
     {% if credentials %}
       <div class="govuk-grid-column-five-sixths">
         {{ page_header(page_title) }}