Add a CollectionMembership class

lib/Exceptions/InvalidCollectionTypeException.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Exceptions;
+
+class InvalidCollectionTypeException extends AppException{
+	/** @var string $message */
+	protected $message = 'Collection type should be `series` or `set` according to the EPUB specification.';
+
+	public function __construct(?string $collectionType){
+		if($collectionType !== null && trim($collectionType) != ''){
+			$this->message .= ' Type provided: ' . $collectionType;
+		}
+
+		parent::__construct($this->message);
+	}
+}