Feature: Update error pages to match modern TOP styling

[Asartea]

Because

• The static error pages haven't been touched for more than a decade, and look wildly out of place among TOP's modern styling

This PR

• Drops 400.html
• Updates 422.html and 500.html to use a modified version of https://tailwindui.com/components/marketing/feedback/404-pages#component-ebef3404d06e7b5f42e85b7b2ca4a2a5, with theme colors taken from TOP's light theme

Issue

Closes #3660

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the keyword: brief description of change format, using one of the following keywords:
• Feature - adds new or amends existing user-facing behavior
• Chore - changes that have no user-facing value, refactors, dependency bumps, etc
• Fix - bug fixes
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• I have verified all tests and linters pass after making these changes.
• If this PR addresses an open issue, it is linked in the Issue section

[KevinMulhern]

Thanks for all your work on this @Asartea, this is a huge improvement 🔥

I've left some feedback, mostly minor styling and wording changes. Happy to discuss any of it.

[Asartea]

Should all be done @KevinMulhern

[Asartea]

I have no clue what is up with that failed test; given that I only made changes in /public I shouldn't be able to touch app code, and Rspec returns no errors when running locally on both main and my branch

[KevinMulhern]

All good @Asartea , it's nothing you've done, just a flaky test on CI - we've been getting them a lot lately 😓.

Rerunning the CI job now, that should sort it out.

[KevinMulhern]

💥 These look great, thank you for taking the time to do them @Asartea 💪