Abort if the instance folder cannot be created

According to the comment, the instance folder should exist in any case.
But a PermissionError was ignored silently.

Since Python 3.9 is the minimum required version, it is safe to use
"exist_ok" added in Python 3.2 and avoid exception handling.

docs/tutorial/factory.rst

@@ -56,10 +56,7 @@ directory should be treated as a package.
             app.config.from_mapping(test_config)
 
         # ensure the instance folder exists
-        try:
-            os.makedirs(app.instance_path)
-        except OSError:
-            pass
+        os.makedirs(app.instance_path, exist_ok=True)
 
         # a simple page that says hello
         @app.route('/hello')