Make ErrorHandler part of the public API

[clue]

This changeset makes the existing ErrorHandler part of the public API. It can now be given explicitly to the App like this:

<?php

require __DIR__ . '/../vendor/autoload.php';

$app = new FrameworkX\App(
    new FrameworkX\ErrorHandler()
);

// Register routes here, see routing…

$app->run();

This is a starting point to add more options to control error handling in follow-up PRs as discussed in #170.

Builds on top of #41, #39, #37, #12