Fix another memory leak by returning the error messages on the stack

Change Song::errorSummary() to return the string with the error messages
on the stack to remove another memory leak.

include/Song.h

@@ -67,7 +67,7 @@ public:
 	void clearErrors();
 	void collectError( const QString error );
 	bool hasErrors();
-	QString* errorSummary();
+	QString errorSummary();
 
 	class PlayPos : public MidiTime
 	{