Respond with 404 Not Found for non-existent tags

What does this MR do?

Handles the case when a non-existent tag is being viewed, and responds with 404 Not Found instead of 500 Internal Server Error.

Are there points in the code the reviewer needs to double check?

The small changes should be clear from a simple diff.

Why was this MR needed?

To resolve the issue #22211 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22211 (closed)