feat: implemented error handling mockup

[sundasnoreen12]

INF-1100

Description

When a user navigates to a discussion board or a discussion post, they see an empty screen with indefinite loading in the following case:

1. They are not enrolled in the course

Acceptance Criteria

We need to show a meaningful message to users instead of indefinite loading.
Figma

Screenshots/sandbox (optional):

Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Sandbox, if applicable.
• Is there adequate test coverage for your changes?

Post-merge Checklist

• Deploy the changes to prod after verifying on stage or ask @openedx/edx-infinity to do it.
• 🎉 🙌 Celebrate! Thanks for your contribution.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f0a4586) 92.76% compared to head (8aa935f) 92.81%.

Files	Patch %	Lines
...c/discussions/discussions-home/DiscussionsHome.jsx	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #663      +/-   ##
==========================================
+ Coverage   92.76%   92.81%   +0.05%     
==========================================
  Files         156      158       +2     
  Lines        3303     3326      +23     
  Branches      905      915      +10     
==========================================
+ Hits         3064     3087      +23     
  Misses        219      219              
  Partials       20       20              

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[awais-ansari]

Please try to remove unnecessary indentation changes.