Change message on unknown db type to be friendly (#17795)
Summary:
CreateDB actually returns nullptr when db type is unknown and throws when the file is missing
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17795
Reviewed By: ezyang
Differential Revision: D14383226
Pulled By: dzhulgakov
fbshipit-source-id: 1dcf75a6b4ba8b64a24d4e5daf02db3189d56b7b
