Fix 500 error for invalid path when visiting blame page

What does this MR do?

Going to a blame URL for a path that doesn't exist used to result in a 500 error. This MR makes it a 404 error.

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

No

Why was this MR needed?

Wrong server error code for the situation was reported.

Does this MR meet the acceptance criteria?

• Changelog entry added
• N/A 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #25761 (closed)