Should return os.ErrNotExist in pkged mode

[Shark]

Thanks @markbates for creating this library. It makes our lives easier! 💙

Version v0.17.1

info, err = pkger.Stat("/idontexist")

When I run this,

• in local filesystem fallback mode (without pkged.go), I get a *os.PathError with syscall.ENOENT as underlying cause (os.IsNotExist returns true)
• with pkged.go at compile time, I get a *errors.errorString saying could not stat github.com/myself/myproject:/idontexist

I feel like using os.IsNotExist is a standard way to find the root cause and handle it appropriately, for example return status code 404 instead of 500.

The code in question is in mem.go.