Fix: forward fatal error in evaluate_condition_for_client()

A fatal error during the evaluation of a condition on behalf
of a client of a notification channel should be forwarded up
the stack to the notification thread.

Here, 'ret' is silently ignored assuming that a check of "evaluation"
is equivalent, which is not the case.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>