Add a try/except around the code to get the files.

The idea is to log the exception but keep going. That way the "good" files still get sent and we can investigate why a file failed.

tests/app/celery/test_letters_pdf_tasks.py

@@ -315,6 +315,48 @@ def test_get_key_and_size_of_letters_to_be_sent_to_print(notify_api, mocker, sam
     ]
 
 
+@freeze_time('2020-02-17 18:00:00')
+def test_get_key_and_size_of_letters_to_be_sent_to_print_catches_exception(
+    notify_api, mocker, sample_letter_template
+):
+    create_notification(
+        template=sample_letter_template,
+        status='created',
+        reference='ref0',
+        created_at=(datetime.now() - timedelta(hours=2))
+    )
+
+    create_notification(
+        template=sample_letter_template,
+        status='created',
+        reference='ref1',
+        created_at=(datetime.now() - timedelta(hours=3))
+    )
+    error_response = {
+        'Error': {
+            'Code': 'FileNotFound',
+            'Message': 'some error message from amazon',
+            'Type': 'Sender'
+        }
+    }
+    mock_head_s3_object = mocker.patch('app.celery.tasks.s3.head_s3_object', side_effect=[
+        {'ContentLength': 2},
+        ClientError(error_response, "File not found")
+    ])
+
+    results = get_key_and_size_of_letters_to_be_sent_to_print(datetime.now() - timedelta(minutes=30))
+
+    assert mock_head_s3_object.call_count == 2
+    mock_head_s3_object.assert_has_calls(
+        [
+            call(current_app.config['LETTERS_PDF_BUCKET_NAME'], '2020-02-17/NOTIFY.REF1.D.2.C.C.20200217150000.PDF'),
+            call(current_app.config['LETTERS_PDF_BUCKET_NAME'], '2020-02-17/NOTIFY.REF0.D.2.C.C.20200217160000.PDF'),
+        ]
+    )
+
+    assert results == [{'Key': '2020-02-17/NOTIFY.REF1.D.2.C.C.20200217150000.PDF', 'Size': 2}]
+
+
 @pytest.mark.parametrize('time_to_run_task', [
     "2020-02-17 18:00:00",  # after 5:30pm
     "2020-02-18 02:00:00",  # the next day after midnight, before 5:30pm we expect the same results