Today, Thiemo merged my patch introducing the ApiErrorReporter class: <https://gerrit.wikimedia.org/r/#/c/124323/>. This should help us with providing error reports from the API in a consistent manner; This way, we will hopefully soon be able to provide more localized error messages too. However, this means that some of the error codes used by the API may have changed, and more will change when more API modules start using this module. Also, it means that localized messages are included in a slightly different way. If you rely on error codes or localized error messages, please keep an eye out for breakage in that regard.