Simplify PSR-4 autoload

classes/Exception/AvconvException.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Alltube\Library\Exception;
+
+/**
+ * Can't find avconv or ffmpeg.
+ */
+class AvconvException extends AlltubeLibraryException
+{
+    /**
+     * AvconvException constructor.
+     * @param string $path Path to avconv or ffmpeg.
+     */
+    public function __construct($path)
+    {
+        parent::__construct("Can't find avconv or ffmpeg at " . $path . '.');
+    }
+}