Close application if not daemon and PyQt not found

src/bitmessagemain.py

@@ -3930,7 +3930,7 @@ if __name__ == "__main__":
         except Exception, err:
             print 'PyBitmessage requires PyQt unless you want to run it as a daemon and interact with it using the API. You can download PyQt from http://www.riverbankcomputing.com/software/pyqt/download   or by searching Google for \'PyQt Download\'. If you want to run in daemon mode, see https://bitmessage.org/wiki/Daemon'
             print 'Error message:', err
-            sys.exit()
+            os._exit(0)
 
         import bitmessageqt
         bitmessageqt.run()