Error handler middleware with error storage

[Fishrock123]

This is http-rs#549 rebased on top of http-rs/http-types#174 & http-rs#570.

Presently does not compile correctly, which makes me a bit concerned about the plan with those two PRs.

error: lifetime may not live long enough
  --> examples\error_handler_middleware.rs:12:33
   |
12 |           |err: &url::ParseError| async move {
   |  _______________-_______________-_^
   | |               |               |
   | |               |               return type of closure is impl std::future::Future
   | |               let's call the lifetime of this reference `'1`
13 | |             let mut response = Response::new(StatusCode::ImATeapot);
14 | |             response.set_body(err.to_string());
15 | |             Ok(response)
16 | |         },
   | |_________^ returning this value requires that `'1` must outlive `'2`

[Fishrock123]

Closing in favor of http-rs@a1cb10e on http-rs#570