Example: https://github.com/bfred-it/iphone-inline-video/blob/gh-pages/this-file-is-gone.md

Current behavior: unhelpful generic 404 page

Expected behavior: show repo that should contain the file/branch (or used to contain it)

Possible additional behavior: if a file/branch was there, use a 410 to show the last know version of it and/or the PR/commit that removed it.