errInvalidUTF8 is not a proto.Error

go.mod

module proto-utf8

go 1.14

require (
	github.com/golang/protobuf v1.4.2
	google.golang.org/protobuf v1.25.0
)

main.go

package main

import (
	"errors"
	"log"
	"proto-utf8/testproto"

	"google.golang.org/protobuf/proto"
)

func main() {
	message := &testproto.Message{
		Str: string([]byte{200, 200, 200}),
	}

	_, err := proto.Marshal(message)
	if err == nil {
		log.Fatalf("expected marshal to fail")
	}
	if !errors.Is(err, proto.Error) {
		log.Fatalf("expected %#v to be a proto.Error", err)
	}
}

test.proto

syntax = "proto3";

package testproto;

message Message {
    string str = 1;
}