+ /**Decodes a custom message to `CustomMessageType`. If the given message type is known to the
+ * implementation and the message could be decoded, must return `Ok(Some(message))`. If the
+ * message type is unknown to the implementation, must return `Ok(None)`. If a decoding error
+ * occur, must return `Err(DecodeError::X)` where `X` details the encountered error.
+ */