salix/back/models/postcode.js

let UserError = require('vn-loopback/util/user-error');

module.exports = Self => {
    require('../methods/postcode/filter.js')(Self);
    Self.rewriteDbError(function(err) {
        if (err.code === 'ER_DUP_ENTRY')
            return new UserError(`This postcode already exists`);
        return err;
    });
};