Error when enable the Export Model Validator

http://sourceforge.net/tracker/?func=detail&aid=2847797&group_id=176962&atid=879332
Add warning message if configured wrongly

base/src/org/adempiere/model/ExportModelValidator.java

@@ -94,6 +94,7 @@ public class ExportModelValidator implements ModelValidator
 		}
 		else 
 		{
+			log.warning("Export Model Validator cannot be used as a global validator, it needs to be defined in a per-client (tenant) basis");
 			return;
 		}