Improve error messages for SSH protocol handshake

If you have an artifact remote cache in your config using the ssh://
protocol and some SSH issue prevents us from connecting, we now show
the user the stderr output from the `ssh` program, e.g.

    [--:--:--] WARNING Failed to fetch remote refs from ssh://artifacts@172.17.0.2:22200/artifacts/: BuildStream did not connect successfully to the shared cache ssh://artifacts@172.17.0.2:22200/artifacts/: Permission denied (publickey,keyboard-interactive).

Previously we would just show the error message raised by the protocol
code, which was less useful, e.g.:

    [--:--:--] WARNING Failed to fetch remote refs from ssh://artifacts@172.17.0.2:22200/artifacts/: BuildStream did not connect successfully to the shared cache ssh://artifacts@172.17.0.2:22200/artifacts/: Expected reply, got none