erp5: make expected errors obvious

balancer instanciation is supposed to fail when zope partitions are not
instanciated yet - add a comment in the failing lines to explain that
it's expected.

stack/erp5/instance-balancer.cfg.in

@@ -129,7 +129,8 @@ ipv6 = {{ zope_address.split(']:')[0][1:] }}
  # As jinja2 postpones KeyError until place-holder value is actually used,
  # do a no-op getitem.
 -#}
-{%   do zope_family_address_list[0][0] -%}
+{%   do zope_family_address_list[0][0] -%} {# This raises IndexError when zope is not instanciated yet, this is expected. -#}
+
 {%   set haproxy_port = next_port() -%}
 {%   set backend_path = slapparameter_dict['backend-path-dict'][family_name] -%}
 {%   do haproxy_dict.__setitem__(family_name, (haproxy_port, zope_family_address_list)) -%}