feat: error pages that say what happened and what to do

The 403, 404 and 500 pages said "Oops!" and one generic sentence; a bad
request and a failed CSRF check fell back to Django's plain pages.

Every error page now shares templates/error.html: a card with the status
in a coloured badge with an icon, a heading in plain words, what happened
and what to do, and Back and Home buttons. It uses only theme-defined
Bootstrap classes, so it reads in the light and dark themes alike.

- 400: the request was not accepted; a new address belongs in ALLOWED_HOSTS.
- 403: the account lacks the permission; ask an administrator.
- 403 CSRF: the form's security check failed; reload and submit again,
  and where to look when it happens on every form.
- 404: the path, and that the object may be gone or hidden from the account.
- 500: a libvirt failure on a page says the host's data could not be
  loaded, under libvirt's message; any other failure says it was logged
  and shows the time to give an administrator.

webvirtcloud/tests/test_connection_errors.py

@@ -66,6 +66,8 @@ class ConnectionErrorPageTests(TestCase):
         response, messages = self.handle(self.admin, libvirt_error(libvirt.VIR_FROM_RPC, HOST_DETAIL),
                                          path="/instances/1/", method="get")
         self.assertEqual(response.status_code, 500)
+        self.assertIn(b"Could not load data from the host", response.content)
+        self.assertIn(HOST_DETAIL.encode(), response.content)
         self.assertTrue(messages)
 
     def test_administrators_see_connection_details(self):

webvirtcloud/tests/test_error_pages.py

@@ -0,0 +1,41 @@
+"""The error pages say what happened and what to do, in the panel's layout."""
+
+from django.contrib.auth import get_user_model
+from django.http import Http404
+from django.test import Client, RequestFactory, TestCase
+from django.views.defaults import page_not_found, server_error
+
+
+class ErrorPageTests(TestCase):
+    def setUp(self):
+        self.user = get_user_model().objects.create_user("ep-user", password="pw")
+        self.client.force_login(self.user)
+
+    def test_a_bad_request(self):
+        response = self.client.get("/", HTTP_HOST="unknown.example")
+        self.assertContains(response, "Bad request", status_code=400)
+        self.assertContains(response, "ALLOWED_HOSTS", status_code=400)
+
+    def test_a_page_the_user_may_not_open(self):
+        response = self.client.get("/computes/")
+        self.assertContains(response, "Access denied", status_code=403)
+        self.assertContains(response, "Ask an administrator", status_code=403)
+
+    def test_a_form_that_fails_its_security_check(self):
+        response = Client(enforce_csrf_checks=True).post("/accounts/login/", {"username": "ep-user", "password": "pw"})
+        self.assertContains(response, "Form could not be submitted", status_code=403)
+        self.assertContains(response, "reload the page and submit the form again", status_code=403)
+
+    def test_a_page_that_is_not_there_names_its_path(self):
+        response = self.client.get("/no/such/page/")
+        self.assertContains(response, "Page not found", status_code=404)
+        self.assertContains(response, "<code>/no/such/page/</code>", status_code=404)
+
+    def test_the_reason_of_a_not_found_is_not_shown(self):
+        response = page_not_found(RequestFactory().get("/datasource/"), Http404("libvirt detail of the host"))
+        self.assertNotIn(b"libvirt detail", response.content)
+
+    def test_a_server_error_renders_without_a_request_context(self):
+        response = server_error(RequestFactory().get("/"))
+        self.assertContains(response, "Server error", status_code=500)
+        self.assertContains(response, "tell an administrator the time", status_code=500)