Return 404 in project issues API endpoint when project cannot be found

What does this MR do?

It fixes project issues API endpoint (both V4 and V3) so that it returns 404 when project cannot be found, instead of crashing.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] 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 #29631 (closed)