Core: Add debug warning when widget not found

See http://stackoverflow.com/a/35713348/145346

js/jquery.tablesorter.js

@@ -1976,6 +1976,8 @@
 						// set priority to 10 if not defined
 						if ( !widget.priority ) { widget.priority = 10; }
 						widgets[ indx ] = widget;
+					} else if ( c.debug ) {
+						console.warn( '"' + names[ indx ] + '" widget code does not exist!' );
 					}
 				}
 				// sort widgets by priority