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.

examples/tutorial/flaskr/__init__.py

@@ -21,10 +21,7 @@ def create_app(test_config=None):
         app.config.update(test_config)
 
     # ensure the instance folder exists
-    try:
-        os.makedirs(app.instance_path)
-    except OSError:
-        pass
+    os.makedirs(app.instance_path, exist_ok=True)
 
     @app.route("/hello")
     def hello():