Derive Default for ErrorMessage

Downstream consumers may have a need to construct an `ErrorMessage`.

Currently, Fuchsia's netlink implementation works around this by
manually constructing the buffer for an `ErrorMessage` then parsing
the buffer[1].

[1]: https://cs.opensource.google/fuchsia/fuchsia/+/main:src/connectivity/network/netlink/src/netlink_packet.rs;l=26;drc=f6e8db007e3fcc51d581f7876a30f1cb4149009f

Author: ghananigans

src/error.rs

@@ -77,7 +77,7 @@ impl<T: AsRef<[u8]> + AsMut<[u8]>> ErrorBuffer<T> {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
 #[non_exhaustive]
 pub struct ErrorMessage {
     pub code: i32,