fix(core): remove UpgradeError from public API

Not making this private was an oversight from an earlier refactoring.

Related: #3915.

Pull-Request: #4012.

core/src/lib.rs

@@ -125,7 +125,7 @@ pub use peer_record::PeerRecord;
 pub use signed_envelope::SignedEnvelope;
 pub use translation::address_translation;
 pub use transport::Transport;
-pub use upgrade::{InboundUpgrade, OutboundUpgrade, UpgradeError, UpgradeInfo};
+pub use upgrade::{InboundUpgrade, OutboundUpgrade, UpgradeInfo};
 
 #[derive(Debug, thiserror::Error)]
 #[error(transparent)]

core/src/upgrade.rs

@@ -69,11 +69,11 @@ mod transfer;
 pub(crate) use apply::{
     apply, apply_inbound, apply_outbound, InboundUpgradeApply, OutboundUpgradeApply,
 };
+pub(crate) use error::UpgradeError;
 use futures::future::Future;
 
 pub use self::{
     denied::DeniedUpgrade,
-    error::UpgradeError,
     pending::PendingUpgrade,
     ready::ReadyUpgrade,
     select::SelectUpgrade,