Add a public function to attach a path to an io::Error

[jplatte]

It would be helpful, for functions not wrapped by fs-err, if I could still attach a path to them easily and get nice-looking messages like provided for wrapped functions.

[andrewhickman]

This would be handy, I'm not sure exactly what the API would look like though. To provide a properly formatted error message, similar to those generated by this library, a description of the operation which failed should be included.