improve error message (#16719)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/16712
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16719
Differential Revision: D13978688
Pulled By: ezyang
fbshipit-source-id: 61f8fa4c54c6969a58550e32e18be2eb9254ced7
