Mark CMZError as non_exhaustive for future extensibility

src/lib.rs

@@ -455,6 +455,7 @@ macro_rules! CMZ {
 
 /// The type for errors generated by the prepare, handle, and finalize
 /// functions generated by the CMZProtocol family of macros
+#[non_exhaustive]
 #[derive(Error, Debug)]
 pub enum CMZError {
     #[error("Hide attribute {1} of credential {0} was not passed to prepare")]