Reader Spaces: show a not-available state when a space returns 404 or 403

[mlaetitia]

Fixes: RSM-4589

Proposed Changes

• Show a "This space isn't available" page when a space returns 404 or 403, instead of a blank page.
• Skip retrying 4xx on the space query so it shows immediately.

Why are these changes being made?

A 404/403 on a space id looked identical to "still loading" and rendered blank forever, with no way out.

Testing Instructions

1. Visit /reader/spaces/<bogus-id>.
2. Expect the not-available message with a "Back to Reader" button; a valid space still loads normally.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• For UI changes, have you tested the affected components in dark mode?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

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[github-actions]

Looks like one of the E2E tests has failed.

You can fix them following these steps:

1. Check out this branch locally:

   gh pr checkout 112341

2. Start Claude Code in the repo:

   claude

3. Run the /fix-e2e-tests skill, passing this PR number:

   /fix-e2e-tests 112341
   

[a8ci18n]

This Pull Request is now available for translation here: https://translate.wordpress.com/deliverables/34237564

Some locales (Hebrew, Japanese) have been temporarily machine-translated due to translator availability. All other translations are usually ready within a few days. Untranslated and machine-translated strings will be sent for translation next Monday and are expected to be completed by the following Friday.

Hi @mlaetitia, could you please edit the description of this PR and add a screenshot for our translators? Ideally it'd include all of the following strings:

• Spaces ‹ Reader
• This space isn’t available
• It may have been removed, or it’s not one of your spaces.

Thank you in advance!