Show error message if precompiled PDF cannot be opened

If PDF files have a validation error which means that they can't be
opened by PyPDF2 we would previously show the 500 status error page. We
now catch PyPDF2.utils.PdfReadErrors so that we can display a custom
error message on the notification page instead.

app/main/views/notifications.py

@@ -24,6 +24,7 @@ from notifications_utils.letter_timings import (
 )
 from notifications_utils.pdf import pdf_page_count
 from notifications_utils.timezones import utc_string_to_aware_gmt_datetime
+from PyPDF2.utils import PdfReadError
 
 from app import (
     _format_datetime_short,
@@ -58,8 +59,14 @@ def view_notification(service_id, notification_id):
     personalisation = get_all_personalisation_from_notification(notification)
 
     if notification['template']['is_precompiled_letter']:
-        file_contents = view_letter_notification_as_preview(service_id, notification_id, "pdf")
-        page_count = pdf_page_count(io.BytesIO(file_contents))
+        try:
+            file_contents = view_letter_notification_as_preview(service_id, notification_id, "pdf")
+            page_count = pdf_page_count(io.BytesIO(file_contents))
+        except PdfReadError:
+            return render_template(
+                'views/notifications/invalid_precompiled_letter.html',
+                created_at=notification['created_at']
+            )
     else:
         page_count = get_page_count_for_letter(notification['template'], values=personalisation)
 

app/templates/views/notifications/invalid_precompiled_letter.html

@@ -0,0 +1,17 @@
+{% extends "withnav_template.html" %}
+
+{% block service_page_title %}
+  Letter
+{% endblock %}
+
+{% block maincolumn_content %}
+
+    <h1 class="heading-large">Letter</h1>
+
+    <p>
+      Provided as PDF on {{ created_at|format_datetime_short }}
+    </p>
+    <p class="notification-status-cancelled">
+      Couldn’t read this file
+    </p>
+{% endblock %}