Facter: Custom-fact-script “was parsed but returned an empty data set”

As far as I know, you can't. Either let it fail, or detect and fail gracefully. I have a number of plugins that only work on Debian that fail on Red Hat without consequence.

Also, please note that it is IMPOSSIBLE to have a fact evaluated AFTER some configuration went in. The architecture just doesn't support it:

Client                               Server
Compute facts
Ask for catalog passing facts =>     Receive Catalog request
                                     Compute catalog using facts
                              <=     Return Catalog
Based on the dependency tree,
  For each configuration with satisfied dependencies
    Apply configuration
    Mark (or not) dependency as satisfied
Send report, if configured so

So, you see, the configuration is only applied long, long after the facts were processed, and there's no way of going back. What may happen is that the next run will now be able to generate that fact.

See also the trick used by the Common module to handle the lack of lsbrelease on Debian without producing fatal errors.

You have a syntax error in:

<%= scope.lookupvar('interfaces').each do |interface| %> <%= scope.lookupvar('ipaddress_'+interface) %>

1. You use <%= when you start iterating: it should be <%
2. You don't close your block

Rewrite like this instead:

<% scope.lookupvar('interfaces').each do |interface| %>
<%= scope.lookupvar('ipaddress_'+interface) %>
<% end %>

Second, interfaces is a String representing all available interfaces on your machine. You need to convert that String into an Enumerable, in which case the #split method comes in handy:

<% scope.lookupvar('interfaces').split(",").each do |interface| %>
<%= scope.lookupvar('ipaddress_'+interface) %>
<% end %>