Return `Result` from constructors which take `TryInto<Url>`

Now that `TryFrom for Url` has been implemented, using the `Url` struct is a lot more pleasant. This opens up the possibility again for us to move from a "panic if parsing fails" to returning errors if parsing fails.

```rust
// current, panic if the url is malformed
let req = Request::post("https://api.foo.com/berries");

// proposed, throw an error instead
let req = Request::post("https://api.foo.com/berries")?;
```