iface: make MulticastError public.

src/iface/interface/mod.rs

@@ -18,6 +18,9 @@ mod ipv6;
 #[cfg(feature = "proto-igmp")]
 mod igmp;
 
+#[cfg(feature = "proto-igmp")]
+pub use igmp::MulticastError;
+
 use core::cmp;
 use core::result::Result;
 use heapless::{LinearMap, Vec};

src/iface/mod.rs

@@ -13,6 +13,9 @@ mod route;
 mod socket_meta;
 mod socket_set;
 
+#[cfg(feature = "proto-igmp")]
+pub use self::interface::MulticastError;
 pub use self::interface::{Config, Interface, InterfaceInner as Context};
+
 pub use self::route::{Route, RouteTableFull, Routes};
-pub use socket_set::{SocketHandle, SocketSet, SocketStorage};
+pub use self::socket_set::{SocketHandle, SocketSet, SocketStorage};