fixup! Add note in design doc, and add _ in Err names

cybozu/validate/design.md

@@ -90,6 +90,11 @@ gRPC implementation. Distinct errors are also generated for each type of
 message which can be used in conjunction with `errors.Is` by callers to 
 take specific actions based the failed validation.
 
+These distinct errors are in the format of 
+`Err<Failed validation>_<Message Name>_<Field NAME>`. For example, failed
+PRECIS username case mapped normalization in message `Strings` field `s4`
+is `ErrPRECISUsernameCaseMapped_Strings_S4`.
+
 [PGV]: https://github.com/bufbuild/protoc-gen-validate
 [go-proto-validators]: https://github.com/mwitkow/go-proto-validators
 [PRECIS]: https://www.rfc-editor.org/rfc/rfc8264.html