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

@param {KeyItem} key
@returns {Integer}
function(key) {
        var num = parseInt(this.loadVar(key));
        if (isNaN(num)) {
            exitProcess('the value of ' +key+' is a necessary int ,but get ' + num, this.alarm);
            return false;
        }
        return num;
    }