googlephotos: don't put an image in error message - fixes #4144

For a certain class of broken or missing image Google Photos puts an
image in the error message.

Before this fix we blindly chucked it into the error message.

After this fix we replace it with some sensible text.

backend/googlephotos/googlephotos.go

@@ -235,6 +235,10 @@ func errorHandler(resp *http.Response) error {
 	if err != nil {
 		body = nil
 	}
+	// Google sends 404 messages as images so be prepared for that
+	if strings.HasPrefix(resp.Header.Get("Content-Type"), "image/") {
+		body = []byte("Image not found or broken")
+	}
 	var e = api.Error{
 		Details: api.ErrorDetails{
 			Code:    resp.StatusCode,