Add pub fn to get a Response from a Rejection

[jtroo]

This is inspired from my desire to easily turn a Rejection into a Response so that my recover handling code is easier. There are some routes where I want to turn all rejections into replies and not fall back to other routes, and I don't want to handle all the built-in rejections manually.

Example:

/// My custom error type
pub enum Error {
    // ... snip
}

impl warp::reject::Reject for Error {}

pub async fn handle_rejection(
    err: Rejection
) -> std::result::Result<warp::reply::Response, std::convert::Infallible> {
    if let Some(e) = err.find::<Error>() {
        // handle my custom rejection
    } else {
        Ok(err.default_response())
    }
}

My original comment:
#712 (comment)

[jtroo]

Could also change this part of the rejections example to use this fn. Any thoughts?

    } else {
        // We should have expected this... Just log and say its a 500
        eprintln!("unhandled rejection: {:?}", err);
        code = StatusCode::INTERNAL_SERVER_ERROR;
        message = "UNHANDLED_REJECTION";
    }

[jtroo]

Just realized that there was another open PR that accomplishes the same thing but using the Into trait: #820

Either one would work for me. The approach in this PR has the benefit of the documentation to warn users about the behaviour of custom rejections.