commands: Added Response

response.go

+package commands
+
+type ErrorType uint
+const (
+  Normal ErrorType = iota // general errors
+  Client // error was caused by the client, (e.g. invalid CLI usage)
+  // TODO: add more types of errors for better error-specific handling
+)
+
+type Response struct {
+  req *Request
+  Error error
+  ErrorType ErrorType
+  Value interface{}
+}
+
+func (r *Response) SetError(err error, errType ErrorType) {
+  r.Error = err
+  r.ErrorType = errType
+}
+
+/*func (r *Response) Encode() ([]byte, error) {
+
+}*/