[4.2] Use FileNotFoundException

`get` or `getRequire` functions both check that the file exists and throw a `FileNotFoundException` if this not the case. So using `files->exists(...)` and thus checking twice that the file exists is useless.
Instead a `FileNotFoundException` can simply be caught.
Use statements should be cascading by length, not sorted alphabetically.

```php
<?php namespace Illuminate\Config;

use Illuminate\Filesystem\Filesystem;
use Illuminate\Filesystem\FileNotFoundException;

class FileEnvironmentVariablesLoader implements EnvironmentVariablesLoaderInterface {