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/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 %}