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

tests/h2-support/src/mock.rs

@@ -1,7 +1,8 @@
 use crate::SendFrame;
 
 use h2::frame::{self, Frame};
-use h2::{self, RecvError, SendError};
+use h2::proto::Error;
+use h2::{self, SendError};
 
 use futures::future::poll_fn;
 use futures::{ready, Stream, StreamExt};
@@ -284,7 +285,7 @@ impl Handle {
 }
 
 impl Stream for Handle {
-    type Item = Result<Frame, RecvError>;
+    type Item = Result<Frame, Error>;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         Pin::new(&mut self.codec).poll_next(cx)