Fix crash when we have a role that we can't match to a barclamp

This should really not happen, but when it happens, we should not
totally fail, and instead keep working so we can provide more
information on what is wrong.

```
ActionView::TemplateError (private method `split' called for nil:NilClass) on line #5 of app/views/nodes/_show_roles.html.haml:
2:   %th
3:     = t("model.attributes.node.roles")
4:   %td{ :colspan => 3 }
5:     = node_role_list(@node)

    app/helpers/nodes_helper.rb:565:in `role_to_proposal_name'
    app/helpers/nodes_helper.rb:456:in `node_role_list'
    /usr/lib64/ruby/1.8/net/protocol.rb:135:in `find'
    app/helpers/nodes_helper.rb:456:in `each'
    app/helpers/nodes_helper.rb:456:in `find'
    app/helpers/nodes_helper.rb:456:in `node_role_list'
    app/helpers/nodes_helper.rb:443:in `map'
    app/helpers/nodes_helper.rb:443:in `node_role_list'
    app/helpers/nodes_helper.rb:442:in `tap'
    app/helpers/nodes_helper.rb:442:in `node_role_list'
```