client.go

package main

import (
	"context"
	"fmt"
	pb "main/errortest"
	"main/hater"
	"reflect"

	"google.golang.org/grpc"
	"google.golang.org/grpc/credentials/insecure"
)

func main() {
	conn, err := grpc.Dial("localhost:9090", grpc.WithTransportCredentials(insecure.NewCredentials()))
	if err != nil {
		return
	}
	defer conn.Close()

	client := pb.NewErrorTestClient(conn)

	_, err = client.ErrorTest(context.Background(), &pb.EmptyMsg{})
	if err != nil {
		fmt.Println(reflect.TypeOf(&err).Elem())

		if h, ok := err.(hat.Hat); ok { //Can't cast to hat.Hat interface
			h.Show()
		}
	}
}

hat.go

package hat

import "fmt"

type Hat interface {
	Show()
	error
}

type CustomError struct {
	Message string
}

func (e CustomError) Show() {
	fmt.Println(e.Message)
}

func (e CustomError) Error() string {
	return e.Message
}

server.go

package main

import (
	"context"
	"fmt"
	"log"
	"net"

	"google.golang.org/grpc"

	pb "main/errortest"
	"main/hater"
)

type ErrorTestServer struct {
	pb.UnimplementedErrorTestServer
}

func (s ErrorTestServer) ErrorTest(ctx context.Context, _ *pb.EmptyMsg) (*pb.StatusMsg, error) {
	return &pb.StatusMsg{}, NewCustomError()
}

func NewCustomError() hater.CustomError {
	return hater.CustomError{
		Message: "Greetings from Bali!",
	}
}

func main() {
	lis, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", 9090))
	if err != nil {
		log.Fatalf("failed to listen: %v", err)
	}
	var opts []grpc.ServerOption

	grpcServer := grpc.NewServer(opts...)
	pb.RegisterErrorTestServer(grpcServer, ErrorTestServer{})

	if err := grpcServer.Serve(lis); err != nil {
		fmt.Print("failed to start server")
	}
}