raise exception if try jit.load non-existent file (#16270)
Summary:
addresses https://github.com/pytorch/pytorch/issues/16267
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16270
Differential Revision: D13791773
Pulled By: suo
fbshipit-source-id: 256304a02dbf724a7c0baade48c94b3ee77f53cf
