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

@param {KeyItem} key
@returns {String}
function(key) {
        var str = this.loadVar(key);
        if (typeof (str) != 'string') {
            exitProcess('the value of ' + key + ' is a necessary string, but get ' + str, this.alarm);
            return false;
        }
        return str;
    }