Context-aware 404 page (BB-2146)

E.g. requesting [[https://bitbucket.org/atlassian/aui-archive/src/27619acd21da/auiplugin/src/doesnotexist|auiplugin-3.6.4 - /src/doesnotexist]] responds with a 404, but what the application could do is recognise that the request is to a sub-path (after the cs hash) and then try to match the path upwards, e.g. auiplugin/src and then auiplugin and then 27619acd21da. The user then gets //redirected// to that path instead of a 404. The page should have a warning message that this is not the originally requested resource but the closest the application could match.