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

Added saver test that asserts correct error when saving to path with parent dir missing

Cleaned up cached files