Usually, message keys of api help, api warning, and api error messages begin with the strings "apihelp", "apiwarn", or "apierror".
This doesn't seem to be the case with the ReadingLists extension, which has keys like readinglists-apierror-project-title-param. As far as I can see, it would be nice to make this consistent and rename them to something like apierror-readinglists-project-title-param, although it's possible that @Tgr, @Nikerabbit, @Raymond, and @Anomie will have a different opinion.
Thanks!