Elmer\Router should have support for custom http error handlers

[peterhorne]

For example:

  <?php
    $app->error(404, function() {
        return array(404, 'Page not found');
    });

Which can then be called inside a route:

  <?php
    $app->get('/', function($app) {
        return $app->error(404);
    });

[peterhorne]

An alternative is to provide implement this as a filter that checks the status code of the response. Example:

<?php
use Elmer\Filters\ErrorHandler;

$errors = new ErrorHandler;
$errors[404] = function($app) {
  return array(404, 'Custom error message');
});

$app->filter($errors);

Routes return a status code with no knowledge of the error handler filter

<?php
$routes->get('/', function($app) {
  return 404;
});