Add missing nested Exception in thrown Exception

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1674618 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/java/org/apache/poi/xssf/model/ThemesTable.java

@@ -45,7 +45,7 @@ public class ThemesTable extends POIXMLDocumentPart {
         try {
            theme = ThemeDocument.Factory.parse(part.getInputStream());
         } catch(XmlException e) {
-           throw new IOException(e.getLocalizedMessage());
+           throw new IOException(e.getLocalizedMessage(), e);
         }
     }