Invalid url should return an error instead of calling panic

When building a request, if one provides a invalid url, e.g., `http://foo.bar:---`, the lib panics. I would expect that the parsing error would be returned at some point.

In my use case the application I have may make requests to links fully provided by the user, and since I can't have the hole app shutting down on a user input error I have to validate the url before passing it to surf's client.

Here's a example of one of these `unwrap` calls that I'm talking about:
https://github.com/http-rs/surf/blob/e9ee6fe54f266e0a099905987ef5c2658d813f91/src/client.rs#L140-L143

One possible way that I think would improve this without needing to change the API would be to have such errors stored in the `Request` object and returned when the struct is eventually evaluated into a `Result`