replace all common error responses with the new Context.StopWithError

Former-commit-id: 99b08a0b5564ef640456db12674cb37721f73ae3

_examples/http_request/read-xml/main.go

@@ -41,8 +41,7 @@ type person struct {
 func handler(ctx iris.Context) {
 	var p person
 	if err := ctx.ReadXML(&p); err != nil {
-		ctx.StatusCode(iris.StatusBadRequest)
-		ctx.WriteString(err.Error())
+		ctx.StopWithError(iris.StatusBadRequest, err)
 		return
 	}