Redirect to 404 if assessment or question is not found

[geshuming]

Issue: currently there is no redirect to 404 when supplying an invalid assessment id or question id.

Example of invalid assessment id: https://sourceacademy.nus.edu.sg/academy/missions/12345/0

Example of invalid question id: https://sourceacademy.nus.edu.sg/academy/missions/16/12345

[chownces]

Addressed in #1878