Fallible services should be more explicit

[Kestrer]

In Hyper, all services that service HTTP requests are currently fallible with any error type that implements Into<Box<dyn Error + Send + Sync>>. However, accepting such a general type makes it prone to mistakes. Usually, servers want to avoid not sending a response, but it can be very easy to accidentally do so with this lax bound. In particular, it would be nice if we could model things like 404 responses as service errors in Axum, but that makes it really easy to accidentally create a service that aborts the connection on a single 404 instead of gracefully sending a response. Additionally, the bound requires that an error type be explicitly chosen for services that don't ever return an error (i.e. most of them), leading to an extra turbofish and Infallible.

Instead, I propose that Hyper only supports services that have AbortConnection as their error type. This type will either be a unit struct or wrap a BoxError. While it would still support services being able to abort the connection if they wish, it is now a lot harder to accidentally do so and requires explicit consent by the user.

[seanmonstar]

Yea, I've had this conversation with others before, here's some bullet point notes I have in a draft roadmap document I'm (slowly) prepping:

• Returning an error in Service::call
  • In HTTP/1, will close the connection
  • In HTTP/2 or 3, reset the stream
    • If the error type is well-known, it can affect the reset stream error code
• Best practice would be to catch all errors and turn them into an appropriate HTTP response
• To encourage best practices, we could require the error type to be stricter
  • Only allow hyper::error::Code
    • Internally can translate to the appropriate HTTP/2 or /3 error code
    • Even HTTP/1 could turn those into the closest HTTP/1 status codes
  • Now a user has to handle their generic application errors, and decide:
    • Since it won't convert automatically
    • Do I convert my error to an error code, or to an HTTP response