In Subversion, treat missing files similarly to conflicted files

Summary: See <https://github.com/facebook/arcanist/issues/133>. Treat "!" files like "C" and "?" files and make the user deal with them.

Test Plan:
  >>> orbital ~/repos/INIS $ svn st
  !       README
  >>> orbital ~/repos/INIS $ arc diff
  Usage Exception: You have missing files in this working copy. Revert or formally remove them (with `svn rm`) before proceeding.

    Working copy: /INSECURE/repos/INIS/

    Missing files in working copy:
      README

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7886