feat(relay): don't close connections upon errors in relay server

protocols/relay/CHANGELOG.md

@@ -1,5 +1,12 @@
 ## 0.17.0 - unreleased
 
+- Don't close connections on protocol failures within the relay-server.
+  To achieve this, error handling was restructured:
+  - `libp2p::relay::outbound::stop::FatalUpgradeError` has been removed.
+  - `libp2p::relay::outbound::stop::{Error, ProtocolViolation}` have been introduced.
+
+  See [PR XXXX](https://github.com/libp2p/rust-libp2p/pull/XXXX).
+
 ## 0.16.2
 
 ## 0.16.1

protocols/relay/src/behaviour.rs

@@ -34,7 +34,7 @@ use libp2p_identity::PeerId;
 use libp2p_swarm::behaviour::{ConnectionClosed, FromSwarm};
 use libp2p_swarm::{
     dummy, ConnectionDenied, ConnectionId, ExternalAddresses, NetworkBehaviour, NotifyHandler,
-    StreamUpgradeError, THandler, THandlerInEvent, THandlerOutEvent, ToSwarm,
+    THandler, THandlerInEvent, THandlerOutEvent, ToSwarm,
 };
 use std::collections::{hash_map, HashMap, HashSet, VecDeque};
 use std::num::NonZeroU32;
@@ -171,14 +171,14 @@ pub enum Event {
     /// Accepting an inbound reservation request failed.
     ReservationReqAcceptFailed {
         src_peer_id: PeerId,
-        error: inbound_hop::UpgradeError,
+        error: inbound_hop::Error,
     },
     /// An inbound reservation request has been denied.
     ReservationReqDenied { src_peer_id: PeerId },
     /// Denying an inbound reservation request has failed.
     ReservationReqDenyFailed {
         src_peer_id: PeerId,
-        error: inbound_hop::UpgradeError,
+        error: inbound_hop::Error,
     },
     /// An inbound reservation has timed out.
     ReservationTimedOut { src_peer_id: PeerId },
@@ -191,7 +191,7 @@ pub enum Event {
     CircuitReqDenyFailed {
         src_peer_id: PeerId,
         dst_peer_id: PeerId,
-        error: inbound_hop::UpgradeError,
+        error: inbound_hop::Error,
     },
     /// An inbound cirucit request has been accepted.
     CircuitReqAccepted {
@@ -202,13 +202,13 @@ pub enum Event {
     CircuitReqOutboundConnectFailed {
         src_peer_id: PeerId,
         dst_peer_id: PeerId,
-        error: StreamUpgradeError<outbound_stop::CircuitFailedReason>,
+        error: outbound_stop::Error,
     },
     /// Accepting an inbound circuit request failed.
     CircuitReqAcceptFailed {
         src_peer_id: PeerId,
         dst_peer_id: PeerId,
-        error: inbound_hop::UpgradeError,
+        error: inbound_hop::Error,
     },
     /// An inbound circuit has closed.
     CircuitClosed {