Ignore a failure to find pam_nologin\.so in SSH PAM configuration

Summary:
This fixes has been introduced to solve T1194, mostly for Debian systems.

FreeBSD, /etc/pam.d/sshd is empty.

Fixes T1865.

Test Plan: Deployed on thrayce

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1865

Differential Revision: https://devcentral.nasqueron.org/D3092