feat: improve extension engine (#298)

1. support onError event
2. add built-in MessageError

pkg/download/engine/inject/error/module.go

@@ -0,0 +1,29 @@
+package error
+
+import (
+	"github.com/dop251/goja"
+)
+
+type MessageError struct {
+	Message string `json:"message"`
+}
+
+func (e *MessageError) Error() string {
+	return e.Message
+}
+
+func Enable(runtime *goja.Runtime) error {
+	messageError := runtime.ToValue(func(call goja.ConstructorCall) *goja.Object {
+		var message string
+		if len(call.Arguments) > 0 {
+			message = call.Arguments[0].String()
+		}
+		instance := &MessageError{
+			Message: message,
+		}
+		instanceValue := runtime.ToValue(instance).(*goja.Object)
+		instanceValue.SetPrototype(call.This.Prototype())
+		return instanceValue
+	})
+	return runtime.Set("MessageError", messageError)
+}