[FR] Improve 404 page when attempting to browse file that does not exist `/browse`

[brendan-kellam]

If I navigate to a path that does not exist when in the code browser, I hit the following error message:

A user could hit this when switching branches, when a file has been deleted or moved from a branch, or other scenarios.

GitHub's 404 page:

[imsarang]

Hi, will look into this issue. @brendan-kellam @msukkari