Add check for directory existence in MD_STD::mkdir

MD_STD.php

@@ -64,6 +64,10 @@ final class MD_STD {
      */
     public static function mkdir(string $pathname, int $mode = 0775, bool $recursive = false):void {
 
+        // One reason, to throw an error, is that the folder already exists.
+        if (\is_dir($pathname)) {
+            return;
+        }
         if (\mkdir($pathname, $mode, $recursive) === false) {
             throw new Exception("Failed to create directory: $pathname");
         }