Show letters that fail validation as cancelled

At the moment we show precompiled letters that have failed the
validation as having been sent. This is confusing.

We should communicate it as having been cancelled (rather than failed),
with the implication being that Notify has come along and cancelled the
letter before printing it. I think this is conceptually what makes the
most sense.

From the user’s point of view any letters that show up as cancelled
probably need to be fixed and resent, so it makes sense to group them
with the same name.

tests/app/main/views/test_activity.py

@@ -547,6 +547,7 @@ def test_big_numbers_and_search_dont_show_for_letters(
         ('sms', 'delivered', 'Delivered 27 September at 5:31pm', True),
         ('letter', 'delivered', '27 September at 5:30pm', True),
         ('letter', 'permanent-failure', 'Cancelled 27 September at 5:31pm', False),
+        ('letter', 'validation-failed', 'Cancelled 27 September at 5:30pm', False),
     ]
 )
 def test_sending_status_hint_does_not_include_status_for_letters(