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.
