Show error when user cannot be archived

A user can't be archived if they are the only member of their service
with `manage_settings` permission. `notifications-api` returns a `400`
and an error message if that is the case, however this PR to remove the
`400` error handler
https://github.com/alphagov/notifications-admin/pull/3320 stopped the
error message from showing. This meant that instead of seeing a message
about why a user couldn't be archived, we would just show a `500` error
page instead. This change checks the response from `notifications-api`
and shows an error banner with a message if the user can't be archived.

tests/app/main/views/test_find_users.py

@@ -4,6 +4,7 @@ import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
 from lxml import html
+from notifications_python_client.errors import HTTPError
 
 from tests import user_json
 from tests.conftest import normalize_spaces
@@ -221,6 +222,39 @@ def test_archive_user_posts_to_user_client(
     assert mock_events.called
 
 
+def test_archive_user_shows_error_message_if_user_cannot_be_archived(
+    platform_admin_client,
+    api_user_active,
+    mocker,
+    mock_get_non_empty_organisations_and_services_for_user,
+):
+    mocker.patch(
+        'app.user_api_client.post',
+        side_effect=HTTPError(
+            response=mocker.Mock(
+                status_code=400,
+                json={'result': 'error',
+                      'message': 'User can’t be removed from a service - check all services have another '
+                      'team member with manage_settings'}
+            ),
+            message='User can’t be removed from a service - check all services have another team member '
+            'with manage_settings'
+        )
+    )
+
+    response = platform_admin_client.post(
+        url_for('main.archive_user', user_id=api_user_active['id']),
+        follow_redirects=True
+    )
+
+    page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+
+    assert normalize_spaces(page.find('h1').text) == 'Platform admin user'
+    assert normalize_spaces(
+        page.select_one('.banner-dangerous').text
+    ) == 'User can’t be removed from a service - check all services have another team member with manage_settings'
+
+
 def test_archive_user_does_not_create_event_if_user_client_raises_exception(
     platform_admin_client,
     api_user_active,