salix/services/loopback/server/middleware/error-handler.js

module.exports = function() {
    return function(err, req, res, next) {
        // MySQL user-defined exceptions are sent to the client
        if (err.sqlState == '45000') {
            err.statusCode = 400;
            err.message = err.sqlMessage;
        }
        next(err);
    };
};