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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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