Non Existing CWD Causes Confusing Error Message

[Aidan63]

nodejs/node#11520

Much like the user in that issue I started questioning my sanity when I started to receive "file not found" errors when writing tests against a program which definitely existing. Turns out it was a bug in my FilePath implementation which resulted in a non-existing cwd path.
The reasons in that issue for not just checking if the file existing seem reasonable so we might want to add to the doc comment that this error will be returned if the specified cwd doesn't existing either.