R1:0ae08b394cd4

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…