pam_systemd: don't use PAM_SYSTEM_ERR for something that isn't precisely a system error

It's not really clear which PAM errors to use for which conditions, but
something called PAM_SYSTEM_ERR should probably not be used when the
error is not the result of some system call failure.

src/login/pam_systemd.c

@@ -660,7 +660,7 @@ _public_ PAM_EXTERN int pam_sm_open_session(
                         return PAM_SUCCESS;
                 } else {
                         pam_syslog(handle, LOG_ERR, "Failed to create session: %s", bus_error_message(&error, r));
-                        return PAM_SYSTEM_ERR;
+                        return PAM_SESSION_ERR;
                 }
         }