Initial import from Alltube app

Removed dependency on Config and LocaleManager classes
Avoid using generic exceptions

classes/exceptions/EmptyUrlException.php

@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * EmptyUrlException class.
+ */
+
+namespace Alltube\Library\Exception;
+
+/**
+ * Exception thrown when youtube-dl returns an empty URL.
+ */
+class EmptyUrlException extends AlltubeLibraryException
+{
+    /**
+     * @var string Error message
+     */
+    protected $message = 'youtube-dl returned an empty URL.';
+}