added to-do comment

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@743932 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/java/org/apache/poi/openxml4j/opc/ZipPackage.java

@@ -250,6 +250,7 @@ public final class ZipPackage extends Package {
 			return new MemoryPackagePart(this, partName, contentType,
 					loadRelationships);
 		} catch (InvalidFormatException e) {
+			// TODO - don't use system.err.  Is it valid to return null when this exception occurs?
 			System.err.println(e); 
 			return null;
 		}