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
You may want to check to make sure this is true, it looks like this is actually true or perhaps there is a bug:

```rust
use std::path::WindowsPath;             
                                        
fn main() { WindowsPath::new(b"\xff"); }
```

```
task '<main>' failed at 'called `Option::unwrap()` on a `None` value', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libcore/option.rs:262
```