salix/back/models/agency-workCenter.js

const UserError = require('vn-loopback/util/user-error');
module.exports = Self => {
    Self.rewriteDbError(function(err) {
        if (err.code === 'ER_DUP_ENTRY')
            return new UserError(`This workCenter is already assigned to this agency`);
        return err;
    });
};