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/buildout.hash.cfg

@@ -86,7 +86,7 @@ md5sum = 5c5f193fb30837327bea361172c5dc96
 
 [template-balancer]
 filename = instance-balancer.cfg.in
-md5sum = f3d38628911a221fdff544452afdb34c
+md5sum = b8764919f5785599079ee81ba265fd6e
 
 [template-haproxy-cfg]
 filename = haproxy.cfg.in

stack/erp5/instance-balancer.cfg.in

@@ -151,7 +151,8 @@ 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)) -%}