Throw a specific exception if MD_STD::mkdir fails

Close #8

src/MD_STD.php

@@ -71,7 +71,7 @@ final class MD_STD {
             return;
         }
         if (\mkdir($pathname, $mode, $recursive) === false) {
-            throw new Exception("Failed to create directory: $pathname");
+            throw new MDFailedToCreateDirectory("Failed to create directory: $pathname");
         }
 
     }