Summary: Ref T13030. See PHI254. This behavior could be cleaner than I've made it, but it fixes the "this is totally broken" issue, replacing a fatal/exception with an informative (just not terribly useful) page. Test Plan: - Added a submodule to a repository. - In Diffusion, clicked some other file next to the submodule, then edited the URI to the submodule path instead. - Before patch: fatal. - After patch: relatively useful message about this being a submodule. Note that it's normally hard to hit this URI directly. In the browse view, submodules are marked up as directories and linked to a separate submodule resolution flow. {F5321524} Reviewers: amckinley Reviewed By: amckinley Maniphest Tasks: T13030 Differential Revision: https://secure.phabricator.com/D18831