Raise a better error for SSH access by users with no permission to use Diffusion
Summary: Fixes T7894.
Test Plan:
$ git pull phabricator-ssh-exec: You do not have permission to access the Diffusion application, so you can not interact with repositories over SSH. fatal: Could not read from remote repository. Please make sure you have the correct access rights and the repository exists.
Reviewers: btrahan, chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T7894
Differential Revision: https://secure.phabricator.com/D12555