Throw error if subdirectory doesn't exist

[vwkd]

The ability to clone a subdirectory with degit user/repo/subdirectory is great!

Currently it seems that it doesn't check if the subdirectory exists in the repo. If it doesn't, it creates an empty target directory which mutates the file system. I'd like to see degit throw an error instead, just like it does if the repo itself doesn't exist.