Throw a specific exception if MD_STD::mkdir fails

Close #8

exceptions/MDFailedToCreateDirectory.php

@@ -0,0 +1,18 @@
+<?PHP
+declare(strict_types = 1);
+
+/**
+ * Reports a failure to create a new directory.
+ */
+final class MDFailedToCreateDirectory extends Exception {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        return 'Failed to encode JSON data.';
+
+    }
+}