Greeter spams system logs if PAM configuration is missing

[dfoxfranke]

cosmic-idle locked my screen in a COSMIC session that I started via start-cosmic. My system was missing /etc/pam.d/cosmic-greeter. This resulted in cosmic-greeter spamming my system logs with the two messages

cosmic-greeter[1187674]: pam_warn(cosmic-greeter:auth): function=[pam_sm_authenticate] flags=0 service=[cosmic-greeter] terminal=[<unknown>] user=[dfranke] ruser=[<unknown>] rhost=[<unknown>]
cosmic-greeter[1187674]: authentication error: Authentication failure

repeated in a tight loop. This tight-looping should be fixed by not retrying a failed PAM call until some sort of user input is received.