Add Result Type for handling Exceptions

[witling]

Idea: Introduce a new class for wrapping up operation results or errors like a sum type.

Old code:

$env = lowebf\Environment::getInstance();
// will throw an exception
$posts = $env->posts()->loadPage(null);
// ...

New code:

$env = lowebf\Environment::getInstance();
// throws an exception like previous example
$posts = $env->posts()->loadPage(null)->unwrap();
// returns the result if found or exits script if not
$posts = $env->posts()->loadPage(null)->unwrapOrExit($env);
// ...

$env->listDirectory(null)
        ->mapOk(function ($folders) {
            // ... do something with folders ...
            return $folders;
        });

Mockup

class Result {
    const OK_STATE = 1;
    const ERROR_STATE = 2;

    /** @var int */
    private $state;
    private $argument;

    private function __construct(int $state, $argument);

    public static function ok($value) : Result;

    public static function error(Exception $e) : Result;

    public function isOk() : bool;

    public function isError() : bool;

    // try to return operations result.
    // throws exception if result is not successful.
    public function unwrap();

    // unwrap the result or use default
    public function unwrapOr($default);

    // clear output buffer and set status code
    public function unwrapOrExit(Environment $env, $statusCode = null);
   
    // if the result is ok -> pass it to $mapper and wrap it in a new result.
    // returns itself otherwise.
    public function mapOk(callable $mapper) : Result;
}

[lausek]

Implemented but needs more testing.