[LMS-2104] improved error message when data set file is not available

SVN: 21455

datastore_server/source/java/ch/systemsx/cisd/openbis/dss/generic/shared/content/PathInfoProviderBasedHierarchicalContent.java

@@ -305,7 +305,7 @@ class PathInfoProviderBasedHierarchicalContent implements IHierarchicalContent
             return asFileContentProvider(node);
         }
         throw new IllegalArgumentException("Resource '" + FileUtilities.getRelativeFile(root, file)
-                + "' does not exist.");
+                + "' is currently not available. It might be in an archive.");
     }
 
     private interface IFileContentProvider