Enable use of ? in Endpoint

[yoshuawuyts]

This PR makes each endpoint return a tide::Result (alias for http_types::Result) which allows the ? operator to work on all endpoints. Effectively this will make unwrap no longer necessary, and will significantly improve our error handling story.

Closes #138 #371 #389.

Example

The Tide "hello world" example now looks like this:

let mut app = tide::new();
app.at("/").get(|_| async { Ok("hello world") });
app.listen("localhost:8080").await?;