Catch cancel_letter errors from API

When we catch such error, if the message is recognised,
show the message and redirect user to view_notification page.

app/main/views/notifications.py

@@ -17,7 +17,7 @@ from flask import (
     stream_with_context,
     url_for,
 )
-from notifications_python_client.errors import APIError
+from notifications_python_client.errors import APIError, HTTPError
 from notifications_utils.letter_timings import (
     get_letter_timings,
     letter_can_be_cancelled,
@@ -185,7 +185,14 @@ def view_notification(service_id, notification_id):
 def cancel_letter(service_id, notification_id):
 
     if request.method == 'POST':
-        notification_api_client.update_notification_to_cancelled(current_service.id, notification_id)
+        try:
+            notification_api_client.update_notification_to_cancelled(current_service.id, notification_id)
+        except HTTPError as e:
+            message_fragments = ["already been cancelled", "too late to cancel"]
+            if e.status_code == 400 and any(fragment in e.message for fragment in message_fragments):
+                flash(e.message)
+            else:
+                raise e
         return redirect(url_for('main.view_notification', service_id=service_id, notification_id=notification_id))
 
     flash("Are you sure you want to cancel sending this letter?", 'cancel')

tests/app/main/views/test_notifications.py

@@ -1,11 +1,11 @@
 import base64
 from functools import partial
-from unittest.mock import mock_open
+from unittest.mock import Mock, mock_open
 
 import pytest
 from flask import url_for
 from freezegun import freeze_time
-from notifications_python_client.errors import APIError
+from notifications_python_client.errors import APIError, HTTPError
 from PyPDF2.utils import PdfReadError
 
 from tests.conftest import (
@@ -958,6 +958,45 @@ def test_cancelling_a_letter_calls_the_api(
     assert cancel_endpoint.called
 
 
+@freeze_time('2016-01-01 15:00')
+@pytest.mark.parametrize('error_message', [
+    "It’s too late to cancel this letter. Printing started on 1 January at 5.30pm",
+    "This letter has already been cancelled",
+    pytest.param("other message", marks=pytest.mark.xfail())
+])
+def test_cancel_letter_catches_errors_from_API(
+    client_request,
+    mocker,
+    fake_uuid,
+    error_message
+):
+    notification = create_notification(template_type='letter', notification_status='created')
+    mocker.patch('app.notification_api_client.get_notification', return_value=notification)
+    mocker.patch(
+        'app.main.views.notifications.get_page_count_for_letter',
+        return_value=1
+    )
+    mocker.patch(
+        'app.main.views.notifications.notification_api_client.update_notification_to_cancelled',
+        side_effect=HTTPError(response=Mock(
+            status_code=400,
+            json=Mock(
+                return_value={'message': error_message}
+            )
+        ))
+    )
+
+    page = client_request.post(
+        'main.cancel_letter',
+        service_id=SERVICE_ONE_ID,
+        notification_id=fake_uuid,
+        _follow_redirects=True,
+    )
+
+    assert page.find('h1').text.strip() == "Letter"
+    assert page.select_one('div.banner-dangerous').text.strip() == error_message
+
+
 @pytest.mark.parametrize('notification_type', ['sms', 'email'])
 def test_should_show_reply_to_from_notification(
     mocker,