Catch all types of exceptions in main routine and show a message box on error...

This MR contains a solution for issue #43 (closed): in the main function a try/catch block has been added to catch all (or most) of the exceptions that might be thrown during the execution of BornAgain. If one is caught, a message box with some details is shown to the user. This way the user see's the details and does not have to look at the console (which is usually absent when run under windows).