function checkArgument(expression, message) {
    if (!expression) {
        throw message;
    }
}
Add a substitution to a feature for a given script and language.
@param {string} feature - 4-letter feature name
@param {Object} sub - the substitution to add (an object like { sub: id or [ids], by: id or [ids] })
@param {string} [script='DFLT']
@param {string} [language='dflt']