Harmonise behaviour of Change reply-to email address

So it acts the same on inital edit and after verification failed.
And also so it does not allow to untick default address in the
latter situation.

app/templates/views/service-settings/email-reply-to/_verify-updates.html

@@ -53,7 +53,11 @@
         width='2-3',
         safe_error_message=True
       ) }}
-      {% if not first_email_address %}
+      {% if form.is_default.data %}
+        <p class="form-group">
+          This is the default reply-to address for {{ current_service.name }} emails
+        </p>
+      {% else %}
         <div class="form-group">
           {{ checkbox(form.is_default) }}
         </div>