Checks if the formatter function should be used.

Throws an error if it is not a function. Returns true if it should be used,
false if no formatting should be performed.

@param formatter
function checkFormatter(formatter, formatterName) {
        if ($.isFunction(formatter)) return true;
        if (!formatter) return false;
        throw new Error(formatterName +" must be a function or a falsy value");
    }