Improve the exception safety code sample

This patch improves the code by showing better handling of the exit
code.

Change-Id: I10f44a93fe342bfd998b717f71ad16216e921d7b
Fixes: QTBUG-71587
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

doc/src/howtos/exceptionsafety.qdoc

@@ -116,13 +116,15 @@
     QApplication app(argc, argv);
     ...
     try {
-        app.exec();
+        int ret = app.exec();
     } catch (const std::bad_alloc &) {
         // clean up here, e.g. save the session
         // and close all config files.
 
-        return 0; // exit the application
+        return EXIT_FAILURE; // exit the application
     }
+    ...
+    return ret;
     \endcode
 
     After an exception is thrown, the connection to the windowing server