// beego filter handler for serve captcha image
func (c *Captcha) Handler(ctx *context.Context) {
	var chars []byte

	id := path.Base(ctx.Request.RequestURI)
	if i := strings.Index(id, "."); i != -1 {
		id = id[:i]
	}

	key := c.key(id)

	if len(ctx.Input.Query("reload")) > 0 {
		chars = c.genRandChars()
		if err := c.store.Put(key, chars, c.Expiration); err != nil {
			ctx.Output.SetStatus(500)
			ctx.WriteString("captcha reload error")
			beego.Error("Reload Create Captcha Error:", err)
			return
		}
	} else {
		if v, ok := c.store.Get(key).([]byte); ok {
			chars = v
		} else {
			ctx.Output.SetStatus(404)
			ctx.WriteString("captcha not found")
			return
		}
	}

	img := NewImage(chars, c.StdWidth, c.StdHeight)
	if _, err := img.WriteTo(ctx.ResponseWriter); err != nil {
		beego.Error("Write Captcha Image Error:", err)
	}
}
// show error string as simple text message.
// if error string is empty, show 500 error as default.
func exception(errcode string, ctx *context.Context) {
	code, err := strconv.Atoi(errcode)
	if err != nil {
		code = 503
	}
	if h, ok := ErrorMaps[errcode]; ok {
		executeError(h, ctx, code)
		return
	} else if h, ok := ErrorMaps["503"]; ok {
		executeError(h, ctx, code)
		return
	} else {
		ctx.ResponseWriter.WriteHeader(code)
		ctx.WriteString(errcode)
	}
}