Refactor errors internals (#556)

h2::Error now knows whether protocol errors happened because the user
sent them, because it was received from the remote peer, or because
the library itself emitted an error because it detected a protocol
violation.

It also keeps track of whether it came from a RST_STREAM or GO_AWAY
frame, and in the case of the latter, it includes the additional
debug data if any.

Fixes #530

src/lib.rs

@@ -104,8 +104,14 @@ macro_rules! ready {
 mod codec;
 mod error;
 mod hpack;
+
+#[cfg(not(feature = "unstable"))]
 mod proto;
 
+#[cfg(feature = "unstable")]
+#[allow(missing_docs)]
+pub mod proto;
+
 #[cfg(not(feature = "unstable"))]
 mod frame;
 
@@ -125,7 +131,7 @@ pub use crate::error::{Error, Reason};
 pub use crate::share::{FlowControl, Ping, PingPong, Pong, RecvStream, SendStream, StreamId};
 
 #[cfg(feature = "unstable")]
-pub use codec::{Codec, RecvError, SendError, UserError};
+pub use codec::{Codec, SendError, UserError};
 
 use std::task::Poll;