Show validation error message for a templated letter over 10 pages

If a letters that has been posted via the API has more than 10 pages it would not get a validation-failed status. This also happens for letters in a CSV upload, only the first row has been validated for having too many pages, because you need to created the pdf before getting an accurate page count.

The API has been updated to mark these letters as invalid and move the
letter to the invalid s3 bucket, the meta data is also set with the
error message and page count.

This PR updates the notification page to display the validation error.

https://www.pivotaltracker.com/story/show/169209742

tests/app/main/views/test_notifications.py

@@ -677,6 +677,29 @@ def test_should_show_image_of_letter_notification_that_failed_validation(
     assert response.get_data(as_text=True) == 'foo', metadata
 
 
+def test_should_show_image_of_templated_letter_notification_that_failed_validation_because_letter_is_too_long(
+    client_request,
+    mocker,
+    fake_uuid,
+):
+    notification = create_notification(
+        notification_status='validation-failed',
+        template_type='letter')
+    mocker.patch('app.notification_api_client.get_notification', return_value=notification)
+    mocker.patch('app.main.views.notifications.get_page_count_for_letter', return_value=11)
+
+    page = client_request.get(
+        'main.view_notification',
+        service_id=SERVICE_ONE_ID,
+        notification_id=fake_uuid,
+    )
+
+    error_message = page.find('p', class_='notification-status-cancelled').text
+    assert normalize_spaces(error_message) == \
+           "Validation failed because this letter is 11 pages long.Letters must be 10 pages or "\
+           "less (5 double-sided sheets of paper)."
+
+
 def test_should_show_preview_error_image_letter_notification_on_preview_error(
     logged_in_client,
     fake_uuid,