What would you like to be added?

Currently WatchResponse message has a string type member CancelReason. The member is used for storing an error which happened during watch.
It's better to add a new Code based member for representing an error in the watch API.

Why is this needed?

Using the existing member for error handling code isn't good because of fragility and hard to track behavior of the etcd wire protocol.

See also:

• Switch vendored etcd to v3.5.6 kubernetes/kubernetes#114403
• [3.5] clientv3: revert the client side change in 14547 #14995
• Remove the error message driven client behavior #14992