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
This is a rather long summary line. I would suggest breaking the two sentences apart and using only the first as the summary. When a summary says that it returns `Some(...)` I think it's reasonable to expect that people will understand that the description explains the conditions under which it returns `None`.