Get value by key, the value must be exist, otherwise the process will exit.

@param {KeyItem} key
@returns {String|Object}
function(key) {
        var value = this.loadVar(key);
        if (typeof(value) =='undefined') {
            exitProcess('the value of ' + key + ' is necessary , but now is undefined', this.alarm);
            return false;
        }
        return value;
    }