feat(mpsc): add `std::error::Error` impls

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

src/mpsc.rs

@@ -89,6 +89,9 @@ impl<T> fmt::Display for Closed<T> {
     }
 }
 
+#[cfg(feature = "std")]
+impl<T> std::error::Error for Closed<T> {}
+
 // === impl TrySendError ===
 
 impl TrySendError {
@@ -146,6 +149,9 @@ impl<T> fmt::Display for TrySendError<T> {
     }
 }
 
+#[cfg(feature = "std")]
+impl<T> std::error::Error for TrySendError<T> {}
+
 // ==== impl Inner ====
 
 impl<N> ChannelCore<N> {