feature: `http::HttpServer::handle_request()` handler closures should return `Result<>`s

**Is your feature request related to a problem? Please describe.**
Currently, the handler closures cannot error out: they must crash on an error.

**Describe the solution you'd like**
It would be nice if there were a way to propagate errors up and out of the closured, e.g. if the closures returned a `Result<()>` and then the `handle_request()` function also returned a `Result<()>`.