When users browse to a submodule path in Diffusion explicitly, don't fatal

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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13030

Differential Revision: https://secure.phabricator.com/D18831