Add message to exception

svn path=/trunk/; revision=360
diff --git a/src/jsr311-api/src/javax/ws/rs/core/GenericEntity.java b/src/jsr311-api/src/javax/ws/rs/core/GenericEntity.java
index 1fb181f..17b5574 100644
--- a/src/jsr311-api/src/javax/ws/rs/core/GenericEntity.java
+++ b/src/jsr311-api/src/javax/ws/rs/core/GenericEntity.java
@@ -74,7 +74,7 @@
    */
   protected GenericEntity(T entity) {
     if (entity==null)
-        throw new IllegalArgumentException();
+        throw new IllegalArgumentException("The entity must not be null");
     this.entity = entity;
     this.type = getSuperclassTypeParameter(getClass());
     this.rawType = entity.getClass();