Add custom exception class for failing downloads

exceptions/updates/MDDownloadException.php

@@ -0,0 +1,25 @@
+<?PHP
+/**
+ * Custom exception class to be raised when downloading a file fails.
+ *
+ * @file
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Custom exception class to be raised when downloading a file fails.
+ */
+final class MDDownloadException extends Exception {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        return 'Link could not be downloaded: ' . $this->getMessage();
+
+    }
+}