mc_api.php:
static function throwIfFault( $p_maybe_fault ) {
if( ApiObjectFactory::isFault( $p_maybe_fault ) ) {
throw new LegacyApiFaultException( $p_maybe_fault->getMessage(), $p_maybe_fault->getCode() );
}
}

Functions getMessage and getCode are used in ApiObjectFactory::throwIfFault but non-existing in class RestFault, so currently a lot of errors in REST API returns Internal Server Error instead of the correct error.