Always include the X-Stream-Error trailer (even without errors)

This is to potentially address things like this:

https://github.com/nodejs/undici/issues/432#issuecomment-1047931107

api/common/api.go

@@ -644,6 +644,10 @@ func (api *API) StreamResponse(w http.ResponseWriter, next StreamIterator, errCh
 
 	if err != nil {
 		w.Header().Set("X-Stream-Error", err.Error())
+	} else {
+		// Due to some Javascript-browser-land stuff, we set the header
+		// even when there is no error.
+		w.Header().Set("X-Stream-Error", "")
 	}
 	// check for function errors
 	for funcErr := range errCh {

api/rest/client/request.go

@@ -156,7 +156,9 @@ func (c *defaultClient) handleStreamResponse(resp *http.Response, handler respon
 	trailerErrs := resp.Trailer.Values("X-Stream-Error")
 	var err error
 	for _, trailerErr := range trailerErrs {
-		err = multierr.Append(err, errors.New(trailerErr))
+		if trailerErr != "" {
+			err = multierr.Append(err, errors.New(trailerErr))
+		}
 	}
 
 	if err != nil {