Add permanent-failure for letters.

It's possible a letter can pass our validation but our print provider can not print the letter. The letter will be marked as permanent failure in this case. Typically happens with precompiled letters.

tests/app/test_model.py

@@ -124,6 +124,7 @@ def test_notification_for_csv_returns_correct_job_row_number(sample_job):
     ('letter', 'created', 'Accepted'),
     ('letter', 'sending', 'Accepted'),
     ('letter', 'technical-failure', 'Technical failure'),
+    ('letter', 'permanent-failure', 'Permanent failure'),
     ('letter', 'delivered', 'Received')
 ])
 def test_notification_for_csv_returns_formatted_status(