Throw an exception if calling a non-existing method on App

[mnapoli]

I've got bitten by this with a project that was compatible with an early beta of Slim 3:

$app->addMiddleware(function (...) {
       ...
});

No error was thrown even though I called a non-existing method because of the specific __call() implementation.

With this change, an error will be thrown when calling an unknown method on App.