lib/promise.js
function (cb) {
            if (cb) {
                if (isPromise(cb)) {
                    cb = cb.errback;
                }
                if (this.__fired && this.__error) {
                    this.__callNextTick(cb, this.__error);
                } else {
                    this.__errorCbs.push(cb);
                }
            }
            return this;
        }
function (cb) {
            if (cb) {
                if (isPromise(cb)) {
                    cb = cb.errback;
                }
                if (this.__fired && this.__error) {
                    this.__callNextTick(cb, this.__error);
                } else {
                    this.__errorCbs.push(cb);
                }
            }
            return this;
        }
function ( cb ) { if ( cb ) { if ( isPromise ( cb ) ) { cb = cb . errback ; } if ( this . __fired && this . __error ) { this . __callNextTick ( cb , this . __error ) ; } else { this . __errorCbs . push ( cb ) ; } } return this ; }
Add a callback to the errback chain of the promise

@param {Function|comb.Promise} cb the function or promise to callback when the promise errors

@return {comb.Promise} this promise for chaining
Add a callback to the errback chain of the promise