Increase rebuilding xref table log message severity up to ERROR

Malformed xref table makes the complete PDF document invalid. Our ability to handle such documents is limited to what we can rebuild, and also this event should be noticed by the users.

DEVSIX-2744

Author: yulian-gaponenko

kernel/src/main/java/com/itextpdf/kernel/pdf/PdfReader.java

@@ -538,7 +538,7 @@ protected void readPdf() throws IOException {
             readXref();
         } catch (RuntimeException ex) {
             Logger logger = LoggerFactory.getLogger(PdfReader.class);
-            logger.warn(LogMessageConstant.XREF_ERROR, ex);
+            logger.error(LogMessageConstant.XREF_ERROR, ex);
 
             rebuildXref();
         }