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

const UserError = require('../../util/user-error');

module.exports = function() {
    return function(err, req, res, next) {
        // Thrown user errors
        if (err instanceof UserError) {
            err.message = req.__(err.message, ...err.translateArgs);
            return next(err);
        }

        // Validation errors
        if (err.statusCode == 422) {
            try {
                let code;
                let messages = err.details.messages;
                for (code in messages) break;
                err.message = req.__(messages[code][0]);
                return next(err);
            } catch (e) {}
        }

        // MySQL user-defined exceptions
        if (err.sqlState == '45000')
            return next(new UserError(req.__(err.sqlMessage)));

        next(err);
    };
};