Warning if no resources have been created

Issue Summary

If you set up everything properly, but don't create any nodes of a resource to be booked, you get a warning on the node edit / add page:

warning: Illegal offset type in /home/drupal5/www/sites/all/modules/resource_conflict/resource_conflict.module on line 223.

If this happens, we should check to see if the current user can create nodes of whatever the referenced types are, and if so, set an error suggesting they create some nodes. If they don't have permission, it should ask them to contact the site email address.

--Andrew