Added code to raise ValueError ifparent directory of path Saver.save() should...

Added code to raise ValueError ifparent directory of  path Saver.save() should use doesn't exist. This give user a much more readable error message than previous NotFoundError

Added code to raise ValueError ifparent directory of  path Saver.save() should use doesn't exist. This give user a much more readable error message than previous NotFoundError

Added some comments

Removed cached file