Include templates for standard error pages

[AdrianDAlessandro]

Description

This PR adds the standard error pages that Django provides views for.

@davehorsfall I looked at the options in the Around template, and I didn't like them as much because they didn't include a navbar and footer, which I think are nice to keep. I also wasn't really able to make them work well with the navbar and footer. So I kept these quite simple. If you have a better approach, feel free to suggest changes, or change them later.

Fixes #640

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Technical work (non-breaking, change which is work as part of a new feature)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. python -m pytest)
• The documentation builds and looks OK (eg. mkdocs serve)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[Sahil590]

Working as expected. LGTM