Small corrections to Open action implementation

A few minor corrections to method execute of class OpenAction:
- get an instance of NSBundle so that is auto-released (which also allows OS X to better re-use it when possible -- according to Apple's docs);
- first check that the app is already running and then log a message about it;
- the success variable does not really make the existing code much more readable or efficient, from my perspective
- set the appropriate errorString value if the call to method openURLs is unsuccessful.