Fix: Only report a general Ajax error when request is complete

- Previously if you cancelled an Ajax request, or the browser did (for
  example following a link) the error handler would be entered and show
  an alert. This just ensures that the request is complete before
  showing an error

media/js/jquery.dataTables.js

@@ -2291,7 +2291,7 @@
 				if ( error == "parsererror" ) {
 					log( oSettings, 0, 'Invalid JSON response', 1 );
 				}
-				else {
+				else if ( xhr.readyState === 4 ) {
 					log( oSettings, 0, 'Ajax error', 7 );
 				}