explicitly loggin an error message for missing system property for string substitution in solrconfig.xml

Solr has a pretty good string substitution function in solrconfig.xml (${system.property[:default value]}). If there is no system property defined, DOMUtil throws a SolrException(500), but it not logged. I'm using Tomcat 5.5.

I had a hard time to find my problem when I forgot to set a system property that was refered in solrconfig.xml.

The patch is to file the problem to the logger explicitly.

regards,