Return 500 error when visit a invalid repo's setting page

**Describe the bug**
When visiting a repo that doesn't exist settings page, it will return a 500 error, as opposed to 404.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to [https://github-clone-dj.herokuapp.com/gitt/APODD/settings/](https://github-clone-dj.herokuapp.com/gitt/APODD/settings/)

**Expected behavior**
Should return a 404 error in some sort

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
- OS: MacOS
- Browser: Chrome
- Version 68

Reproduce the problem at the settings URL shown in the issue using a repository that does not exist, then trace the request handling for that page. Confirm the current 500 response and identify the existing not-found behavior to follow. Done means the invalid repository settings URL returns a 404 instead of a 500.

Written by the indexing model from the issue text.