Fix error message for Path::new

Originally discovered here: http://www.reddit.com/r/rust/comments/2dbg3j/hm_unwrap_is_being_renamed_to_assert/cjnxiax
`WindowsPath::new()` requires UTF-8. `PosixPath::new()` only requires no NUL bytes. `Path::new()` may fail in either scenario depending on current platform.