Error message for previews is confusing if they are loaded with a delay

Just recognised this due to a slow internet connection: if it takes some time to load the files/previews, the error message "file not found" shows up immediately. This is really confusing. It looks as if my Timeline is broken (same in Post-Annotator). I guess we could just extend the time in which it still tries to get the asset and show a spinning wheel or another loading indication?