browse: Improve display of 404 errors for content and directory

When trying to browse a content or a directory in another object context
(snapshot, revision, directory), if the targetted object can not be found
(for instance after changing the current branch in a snapshot) display a
404 error while keeping display of context related UI to ease the navigation.

Also improve the displayed error message when a directory entry with a
given path can not be found from a root directory.

Closes T2353