better default exception check in hook

[richardo2016]

// valid check, hook would throw exception
beforeCreate: function (next) {
    let err = null

    err = someCheck()
    if (err && err.message)
        return next(new Error(err.message))
}
// invalid check, hook would throw one lonely message 'undefined' only
beforeCreate: function (next) {
    let err = null

    err = someCheck()
    if (err && err.message)
        return next(err.message)
}