Add some summaries of letter validation errors

We show letter validation errors in two places:

1. In response to a user uploading a PDF
   Here we use the error banner pattern because the problem is as a
   direct consequence of a user’s action, and is blocking them from
   continuing.

2. Once a PDF provided through the API has been validated
   We use a less prominent pattern of red text with no border because
   the message is reporting on something that’s already happened, and
   which wasn’t a direct consequence of the user clicking something

Because the context and patterns used are different we need slightly
different content in each of these situations. Previously we tried to
reuse the same content to make the code cleaner and less repetitive. But
ultimately a clear interface trumps clear code.

app/templates/views/notifications/notification.html

@@ -44,7 +44,7 @@
         </p>
       {% elif notification_status == 'validation-failed' %}
         <p class="notification-status-cancelled">
-          Validation failed. {{ message.detail | safe }}
+          {{ message.summary | safe }}
         </p>
       {% elif notification_status == 'technical-failure' %}
         <p class="notification-status-cancelled">