UHF-10060: Fix system error pages

[teroelonen]

UHF-10060

What was done

• This PR adds templates for the system error pages 401, 403 and 404. They are already correctly displayed in the case of a real error but these pages in the path /system/[error-number] can in some cases be seen by users and in those situations the layout is broken.

How to install

• Make sure your instance is up and running on latest dev branch.
  • git pull origin dev
  • make fresh
• Update the HDBT theme
  • composer require drupal/hdbt:dev-UHF-10060
• Update the helfi_platform_config
  • composer require drupal/helfi_platform_config:dev-UHF-10060
• Run make drush-cr

How to test

• Go to pages /system/401, /system/403 and /system/404 and make sure the layout is not broken on those pages anymore but instead they display the same kind of error page as one would except when encountering an error on the website.
• Go to some url that will definitely return 404 error such as good old /asdf and make sure that you are not able to switch languages on the page anymore.
• Log out and try going to some url that is behind login and you should get 403 error. On this page you shouldn't be able to change language from the language switcher anymore either.
• Check that code follows our standards

Continuous documentation

• This feature has been documented/the documentation has been updated
• This change doesn't require updates to the documentation

Other PRs

• UHF-10060: Fix system error pages #1005
• UHF-10060: Fix system error pages drupal-helfi-platform-config#771

[github-actions]

⚠️ Visual regression found! Please check if this change is wanted or accidental. You can check the output here: https://city-of-helsinki.github.io/drupal-hdbt/pull/1005/html_report/

[Arkkimaagi]

Nice, clean, works as it should! Now also Approved! 🌹