fix(s2n-quic-transport): Map poll_accept errors to documented return value

[maddeleine]

Resolved issues:

resolves #1712

Description of changes:

This change fixes our code so that our "accept" functions return what our documentation says they return. Previously they could return both Ok(None) and Err(connection::Error::Closed) to indicate the connection has been closed. Now they return Ok(None) for all errors that indicate the connection was closed.

Call-outs:

Used the #[inline] attribute because this function is small and is called in a different crate than it's defined. I think that's how it's supposed to be used?

Testing:

I manually tested that this change fixed the issue for connection::Error::Closed and connection::Error::IdleTimerExpired.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[harrisonkaiser]

🥳