package main

import (
    "errors"
    "fmt"
    "io"
    "net/http"
    "strings"

    "golang.org/x/net/websocket"
)

func main() {
    echo := websocket.Server{
        Handshake: func(config *websocket.Config, req *http.Request) error {
            return errors.New("handshakeError")
        },
        Handler: websocket.Handler(func(ws *websocket.Conn) {
            io.Copy(ws, ws)
        }),
    }
    http.Handle("/echo", echo)

    go http.ListenAndServe(":12345", nil)

    _, err := websocket.Dial("ws://localhost:12345/echo", "", "http://localhost/")
    if err == nil {
        fmt.Fatal("except failed.")
        return
    }

    if strings.Contains(err.Error(), "handshakeError") {
        fmt.Println("ok") // error message is contains excepted string 'handshakeError'
    } else {
        fmt.Println("failed") // error message don.t contains excepted string 'handshakeError'
    }
}