Serve simple error pages if HTML isn't required

[RealOrangeOne]

Description of Changes Made

Our 404 page is a fancy HTML page, comprised of multiple templates, and requiring a number of DB queries to create (not many queries, granted). If a person in a browser loads a page, we want to show them this "fancy" 404 page for a better user experience. However, if the request shouldn't return HTML (eg it's a missing static file) or user never asked for HTML, we shouldn't spend the time creating a fancy 404 page if it's never going to be viewed.

Instead, when possible, we show a simplified HTML page, which just contains text. This requires much fewer resources to generate, and is quicker to serve.

How to Test

If you go to a missing page with curl, you should just get some text, rather than a full HTML page.

MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Tests and linting passes.

Unit tests

• Added
• Not required

Documentation

• Updated build docs
• Updated editor guidelines (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only)
• Updated field spec (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only)
• Not required

Browser testing

• I have tested in the following browsers and environments (edit the list as required)
  • Latest version of Chrome on mac
  • Latest version of Firefox on mac
  • Latest version of Safari on mac
  • Safari on last two versions of iOS
  • Chrome on last two versions of Android
• Not required

Data protection

• Not relevant
• This adds new sources of PII and documents it and modifies Birdbath processors accordingly

Light and dark mode

• I have tested the changes in both light and dark mode
• The change is not relevant to dark and light mode

Accessibility

• Automated WCAG 2.1 tests pass
• HTML validation passes
• Manual WCAG 2.1 tests completed
• I have tested in a screen reader
• I have tested in high-contrast mode
• Any animations removed for prefers-reduced-motion
• Not required

Sustainability

• Images are optimised and lazy-loading used where appropriate
• SVGs have been optimised
• Performance and transfer of data considered
• If JavaScript is needed alternatives have been considered
• Not required

Pattern library

• The pattern library component for this template displays correctly, and does not break parent templates
• The styleguide is updated if relevant
• Changes are not relevant the pattern library