Provide a more helpful message if a user connects via raw SSH

Summary: We currently print a fairly vague, technical message which is ambiguous about indicating success or an error if you aren't familiar with SSH.

Test Plan:
  $ ssh -T dweller@localhost
  phabricator-ssh-exec: Welcome to Phabricator.

  You are logged in as epriestley.

  You haven't specified a command to run. This means you're requesting an interactive shell, but Phabricator does not provide an interactive shell over SSH.

  Usually, you should run a command like `git clone` or `hg push` rather than connecting directly with SSH.

  Supported commands are: conduit, git-receive-pack, git-upload-pack, hg, svnserve.

Reviewers: btrahan, dctrwatson

Reviewed By: dctrwatson

CC: aran

Differential Revision: https://secure.phabricator.com/D7854