quest(cpp): C++ errors carry their message

quest/m1/cpp/README.md

@@ -51,6 +51,7 @@ reads the same release manifest so a release bumps both.
 ## Quests
 
 - [Package](/quest/m1/cpp/package.md) - the `cpp/moq` wrapper, CMake package, release tarball, interop client, and docs
+- [Error messages](/quest/m1/cpp/error-message.md) - a C++ `moq::Error` prints the same message Rust gives
 - [OBS migration](/quest/m1/cpp/obs.md) - the OBS plugin moves from libmoq handles and trampolines to the generated C++
 
 ## Related

quest/m1/cpp/error-message.md

@@ -0,0 +1,21 @@
+# [XS] C++ errors carry their message
+
+## Goal
+
+A `moq::Error` from the C++ package prints the same message Rust's `Display`
+gives, so callers and the interop client log a reason rather than a variant
+name.
+
+## Plan
+
+- The generator emits no `Display` for errors. Prefer exposing the message
+  through moq-ffi once (e.g. a method on the error the generator already
+  binds) over hand-written per-variant strings in the wrapper.
+- Use it in the interop client and the doc samples.
+
+Public API: additive on the C++ package, and on moq-ffi if a method is added.
+Wire: none.
+
+## Related
+
+- [#4187](https://github.com/moq-dev/moq/pull/4187) - the package that found it