Fix 404 status header

[Zifius]

Description

Changes 404 header status text to "Not Found" instead of wrong "Forbidden"

Fixed Issues (if relevant)

1. 404 Forbidden sounds not right #10775: 404 Forbidden sounds not right

Manual testing scenarios

1. Go to Catalog -> Products and click "Add Product" button
2. On the page opened remove "set/4" from URL and hit enter
3. In developer tools console observe "Not Found" hint when hovering 404 in the "Status" column

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)